#What is the Is/Ought Problem?

The "is/ought problem," also rarely known as "Hume's guillotine," "Hume's law," etc. is a point made by the philosopher David Hume. Hume, in the process of objecting to moral theories that disagreed with his own moral theory, suggested that many moral philosophers provide arguments that spend a lot of time talking about how the world is, and then at one point start talking about how the world ought to be, without ever making it clear how we get from the first sort of statement to the second sort of statement. At least on their surface, the two sorts of statements seem very different. The first sort of statements, "is" statements, describe things in what we might call a "non-normative" sense, which means that they aren't directly about how anyone should act, or about what would be better or worse, or anything like that. They aren't directly about morality, in other words. The second sort of statements, "ought" statements, are "normative" - they judge how things should be, or what we ought to do, or what would be better or worse.

There's some controversy over exactly what Hume meant to suggest by pointing out this gap between "is" and "ought" (for discussion, see here). But one thing is clear: his point is that the two sorts of statements seem very different, and so it's hard to see how we can draw conclusions about normative statements merely from non-normative statements. Anyone who is trying to do this owes us some kind of explanation, which Hume thinks the other philosophers hadn't provided. This is the "is/ought problem." How do we get from "is" statements to "ought" statements?

There are, broadly speaking, two sorts of replies to the is/ought problem. The first sort of reply gives a recipe for moving from "is" to "ought." The second sort of reply says that there's no way to avoid the problem, and so moral arguments need to start with some "ought" statements, too, instead of starting only with "is" statements.

#Reply 1: Jump the Gap

Some philosophers argue that we can cross the is/ought gap. The simplest argument is one given by John Searle in the paper How to Derive an "Ought" from an "Is". He suggests that we can cross from "is" statements to "ought" statements in (for instance) arguments like this:

1. Mack says "I promise to pay Blaine $5 on Tuesday."

2. Mack has promised to pay Blaine $5 on Tuesday.

3. Mack has undertaken an obligation to pay Blaine $5 on Tuesday.

4. Mack is under an obligation to pay Blaine $5 on Tuesday.

5. Mack ought to pay Blaine $5 on Tuesday.

This argument seems straightforward. Moreover, most of those statements seem like "is" statements. Maybe they are all "is" statements except the fifth. But the fifth is definitely an "ought" statement. So at some point it must be possible to move from "is" statements to "ought" statements without doing anything wrong. Problem solved!

This is literally the simplest response to the "is/ought" problem, which is why I reproduce it in full here. Typical responses that jump the gap are much more detailed. They go on for many pages. For a very good example see chapter 6 of Brink's book Moral Realism and the Foundation of Ethics, which is another attempt to jump the is/ought gap. Broadly speaking, lots of projects in metaethics contain attempts to jump the is/ought gap. There is disagreement about whether any of them succeed.

#Reply 2: Start with Oughts

Other philosophers accept Hume's argument and thus build morality not merely on "is" statements but also on one or more "ought" statements. This is one typical view about what Hume himself did. Hume (the way many people read him) identified the is/ought gap and thus built a moral system that also has a few "oughts" in the premises, so that we can derive "oughts" in the conclusions. Whether or not this is a good description of Hume, it's definitely a good description of many other philosophical views. For instance, many read Kant's moral philosophy as being built on at least one "ought" statement.

#Does the Is/Ought Gap Lead to Moral Skepticism?

Some people erroneously think that the is/ought gap, if it exists, shows that morality must be a joke. The is/ought gap, according to this understanding, disproves the possibility of anything being morally wrong. Either we reject the gap or we reject morality. Obviously it should be clear why this is incorrect: as noted above, the second sort of reply accepts the is/ought gap, but hardly gives up on morality. Many people on reddit have this erroneous view because Sam Harris believes it and Sam Harris is popular on reddit. Sam Harris, however, badly misunderstands the is/ought gap. For a description of his misunderstanding in detail, see this series of posts by /u/wokeupabug: one, two, three.

The closest thing to this sort of view which is philosophically respectable is the view that the is/ought gap motivates non-cognitivist views in ethics. Some people think Hume was a non-cognitivist, for instance. But non-cognitivism is distinct from moral skepticism, and the view that Hume was a non-cognitivist is not a very common one. In any case, the is/ought gap is not supposed to be an argument for moral skepticism. It is an argument for an approach to morality which either motivates the jump from "is" to "ought," or which starts with one or more "oughts," or which does not rely on the notion of "oughts" at all.

#More Reading

https://www.reddit.com/r/askphilosophy/comments/2tkq32/responses_to_humes_guillotine/

http://www.reddit.com/r/askphilosophy/comments/1slgqd/can_a_proposed_system_of_objective_ethics_still/

http://www.reddit.com/r/askphilosophy/comments/1wmmm5/challenge_to_the_isought_distinction_based_on_the/

http://www.reddit.com/r/askphilosophy/comments/2sivxx/isought_problem/

http://www.reddit.com/r/askphilosophy/comments/1op3o1/what_are_the_usual_responses_to_the_isought/

http://www.reddit.com/r/askphilosophy/comments/2iw52b/how_do_moral_objectivistsrealists_respond_to_the/

https://www.reddit.com/r/askphilosophy/comments/4hute0/is_there_a_good_rebuttal_to_humes_is_ought_problem/

https://www.reddit.com/r/askphilosophy/comments/4uc335/isought_problem_responses/

https://www.reddit.com/r/askphilosophy/comments/czbb3c/has_there_been_an_indepth_rebuttal_to_humes/

https://www.reddit.com/r/askphilosophy/comments/d33f57/problems_with_the_isought_fallacy/?st=k51q53xm&sh=2ff3ad11

https://www.reddit.com/r/askphilosophy/comments/ekk8wn/has_humes_guillotine_ever_been_credibly_refuted/

#1. Introduction

    Panelists^(1) and scholars (including even moral relativists) often make the following sorts of claims about moral relativism: it is unpopular^(2), uncommon^(3), extremely unpopular^(4), fringe^(5), untenable^(6), almost always a criticism^(7), and, in some noteworthy forms, straight up incoherent^(8). At the same time, moral realism appears to hold only a slim majority. Moral anti-realism, on the other hand, is supported by 27.8% of metaethicists.^(9) 27.8% obviously isn't fringe or extremely unpopular. Further, panelists have claimed that those who take morality to be subjective have pretty significant representation^(10) in the contemporary literature.

Suppose someone comes to /r/askphilosophy. They believe that whatever is moral is arbitrarily decided and is all a matter of mere opinion. In other words, they believe notions like 'evidence for moral facts' and 'arguments for some thing being morally wrong' are nonsensical. This is actually a common belief. For this person, the situation I've described in the first paragraph can be confusing. It's difficult for them to tell if their view is fringe or not with all this seemingly contradictory information. But in fact, there is no contradiction at all, for the terms "moral relativism," "moral anti-objectivism," and "moral anti-realism" should all be distinguished from one another.

This submission will be primarily concerned with what seems to be a conflict. Some claim that subjective groundings of ethics are prominent among metaethicists. Some claim that relative groundings of ethics don't have much currency among metaethicists at all. I'll go over what some relevant terms can be taken to mean, what that entails, and why that's significant, thus laying out the distinctions between them.

The rest of it will address the conflict between relativity being fringe and the survey showing anti-realism's significant representation by showing that these terms are different, and thus no actual conflict exists. This is a similar, but distinct topic from my main focus for reasons I'll be making clear below.

This will all be followed by a very brief summary to help consolidate everything presented here, make it easier to read along, and to let anyone who simply wants a conclusion have easy access to it.

#2. Relativism: An apparent conflict

##2.1 What 'Subjective Facts,' 'Objective Facts,' 'Relative Facts,' and 'Absolute Facts' Are

    The meaning of subjective and, conversely, objective for the aforementioned panelists is important to understand here but notoriously difficult to clarify. We can point at some sentences that seem to be objectively true and gesture at others that seem to be subjectively true.

Both of these sentences appear to be objectively true:

• The mass of the Sun is over three hundred thousand times the mass of the Earth.
• The climate of the Earth is warming.

On the other hand, here's a sentence that might be subjectively true:

• This device I'm reading on is worth the same as these two thousand similar sheets of paper.

The difficult part is making explicit what it is that makes facts about the Sun's mass and the Earth's climate objective and facts about currency subjective. It is often said that subjectivity is "mind-dependence," so a subjective fact is a true sentence whose truthness depends on a mind(s) or mental activity. However, the definition cannot end there. It's clear that many objective facts would be considered subjective under this conception of subjectivity, including one of the examples above. On this conception, we'd have to accept that the sentence "the climate of the Earth is warming" being true is subjective. Its truthness depends on human activity, and thus depends on mental activity.

We're not interested in a mere cause-effect relationship between our thoughts and the world when we talk about subjectivity and objectivity. One way of understanding subjectivity that lets us define it as more than mere dependence on mental activity is stance-dependence.

Put simply, a fact is stance-dependent if it is true by virtue of its acceptance from within some point of view (whether actual or hypothetical)^(11). So, that the climate is changing is objectively the case, but it is the case in spite of the mental activity involved in such a thing being true. It is true, but not by virtue of its acceptance from within some point of view. We could even have everyone, every point of view, reject that the climate is changing and it would still be true that the climate is changing. This way of understanding subjectivity really seems to fit the bill and lets us point out a lot of matters that are objective and others that are subjective.

So, when someone says "morality is subjective" or "moral facts are subjectively true," what they are saying is adequately understood as "moral facts are true by virtue of their acceptance from within some point of view." Conversely, when someone says "morality is objective" or "moral facts are objectively true," a good way to interpret that is "moral facts are true, but not by virtue of their acceptance from within some point of view."

Are these the same as when someone claims, respectively, that "morality is relative" or "morality is absolute?" As the aforementioned panelists understand it, no. If we say some sentence S has a relative truth value, we are saying that it is possible for S to be true or false without S being so for everyone. So, Jordan and Chris can say S is true, and Jordan can be correct while her friend, Chris, is incorrect. In this case, S is true-for-Jordan and false-for-Chris, so when Chris says S true, she's wrong. If Jordan says S is true while Chris says S is false, they are both correct.

If we say some other sentence s is absolutely true, then Bryce and her uncle Shannon can't disagree and both be correct. Nor can they agree on s and only one be wrong. They are in the same boat, so if s is true, it is true for both of them. If s is false, it is false for both of them.

A good way of interpreting the statement that "morality is relative" or "moral facts are relatively true," then, is "it is possible for the moral sentences which are true for this individual or group to be false for a different individual or group." Conversely, we should understand the claim that "morality is absolute" or "moral facts are absolutely true" to mean that "it is impossible for the moral sentences which are true for this individual or group to be untrue for any other individual or group."

##2.2 The Independence of Subjectivity

    What this entails is that subjectivity is distinct from relativity. There are multiple senses in which that statement is true. Subjectivity is distinct from relativity in that they literally have different definitions. No reasonable person would contest that the definitions given above for subjectivity and relativity are different from one another, but this is a very uninteresting distinction.

What's more noteworthy is that subjectivity and relativity are independent from one another. In other words, a sentence being subjectively true does not conceptually entail that it is relatively true, nor does a sentence being relatively true entail that it is subjectively true. Similarly, a sentence being objectively true does not entail that it is absolutely true, nor does a sentence being absolutely true entail that it is objectively true.

We can demonstrate both of these distinctions with some examples^(12 13):

1. Suppose that every true epistemic sentence (a sentence relating to knowledge) is true in virtue of Cameron believing they are true. Take the epistemic sentence 'everyone ought to proportion her belief to the evidence.' Suppose this is true. What explains it being true? The fact that Cameron believes it.

What is this 'in virtue of' relation? Consider the fact that 7 is prime.

• What explains 7 being prime is what prime number are and what 7 is. Prime numbers are integers greater than 1 whose only natural number factors are 1 and itself. 7 is an integer greater than 1 whose only natural number factors are 1 and 7.

  '7 is prime' is true in virtue of what prime numbers are and what 7 is.

• What doesn't explain 7 being prime is the fact that Samus Aran is one of Nintendo's smartest characters.

  '7 is prime' is still true, but not in virtue of Samus Aran's intelligence.

Similarly, 'everyone ought to proportion her belief to the evidence' is true in virtue of Cameron believing it is true.

Now, we can also imagine that Cameron's belief cannot change. It's simply conceptually impossible. Here, true epistemic sentences are absolutely true; they are true for everyone, everywhere, always. Nonetheless, they are also subjectively true; the reason they're true is some mind thinking they are true. In the case I've just described, true epistemic sentences are absolutely and subjectively true.

2. Heaviness is relative and objective. Phoenix, who is a person of average weight, may be heavy in relation to some individuals or groups (e.g. children) but light in relation to other individuals or groups (e.g. sumo wrestlers), so a sentence about her/his heaviness can be true-for-some and false-for-others. Nonetheless, no stance anyone has is relevant to these truth-values. So, if Phoenix goes to sign up for sumo wrestling and the sumo wrestlers say "you're heavy!" then they're all wrong. For them, Phoenix is light, and this fact is stance-independent, or objective.

In public discussion, one common objection to cases like Cameron's looks something like "Wait, neither Cameron nor Cameron-like entities exist in real life! So this doesn't demonstrate that subjectivity is distinct from relativity at all since it's completely unrealistic." This objection is incredibly pervasive regardless of how Cameron's case is being used, so we'll go over how this objection isn't really relevant in multiple usages:

• Sometimes, this objection is used to show how the definition of subjective and relative are literally distinct. The accusation towards Cameron that their existence is unrealistic makes no sense here whatsoever. They don't need to exist to simply show that the words themselves are distinct. If I note that all true squares (which are proposed as abstract objects) can fit into true square holes that equal their height whereas not all true rectangles can fit into true square holes that equal their height, I've demonstrated that true squares are not the same thing as true rectangles even if someone chimes in with "but true squares and rectangles don't really exist!"

• Other times, the objection is used to show that subjectivity, in actuality, entails relativity. The objection here is that Cameron doesn't exist in real life so subjective facts are, in reality, always relative facts. However, this misunderstands what Cameron is trying to show us.

  Cameron is not trying to make you conclude that they exist, they are far too shy and modest for that^(14). Cameron is trying to show that there is no conceptual reason subjectivity should entail relativity. So, bringing back a previous example, there is no similar case we can think of for true squares^(15). The very concept of a true square entails that it is also a true rectangle.

  Contrarily, there's no part of subjectivity, as a concept, that means we're also dealing with relativity, and Cameron demonstrates this very effectively.

At any rate, Phoenix's case does provide a realistic example of a fact's being objective not entailing that it is also absolute, so we can conclude that at the very least, the objective-subjective distinction is in some significant ways actually independent from the absolute-relative one.

##2.3 The Absolute Dependability of Stances

    The significance in all of this is if subjectivity and relativity are distinct, then the following would be consistent: very few experts claiming that morality is relative; very many experts claiming that morality is subjective.

There are many ways to coherently claim that morality is subjective and absolute.

• We can imagine someone dispassionate, disinterested, and ideally reasonable watching all our affairs and interactions, aware of everything going on. If it's the case that the moral claims which are true for everyone are the moral claims that she would accept, then morality is both absolute and subjective. She is not an actual person, she is entirely hypothetical, but recall that stance-dependence does not require that the point of view is actual. Some ideal observer theorists^(16) claim that this is the case, and so would affirm that morality is subjective, but since only one point of view matters here, whatever is true for me is also true for you.

• Perhaps it’s the case that a moral claim is true if an agent judges it to be true in reflective equilibrium^(17). If that’s the case, then morality is stance-dependent. It is dependent on the stances she would have on these propositions if she were in reflective equilibrium, which is a hypothetical point of view. If, as some constructivists will claim, her stances in reflective equilibrium are the stances anyone, anywhere, anytime^(13) would have in reflective equilibrium, then which moral propositions are true is an absolute and subjective matter.

The question of why there's such a strong academic consensus against moral relativism and what moral relativisms are taken seriously is not within the scope of this submission and will require further reading^(11 18 19 20 21), though it should be briefly noted here that the relativisms that are taken seriously look nothing like "morality is just arbitrary opinions^(19)."

#3. Anti-realism: Also an apparent conflict

    So it would appear we've solved the issue! This submission is over and the text you see below is a hallucination on your part. It seems like most metaethicists are moral realists and think that morality is objective, but a fourth of metaethicists are moral anti-realists, and so think that morality is subjective, not objective. The reason relativism can still be fringe, then, is that those who think morality is subjective are absolutists, not relativists.

However, this story is wrong. It only (rather inaccurately) captures half^(22) the story, so we'll fill in the rest below.

##3.1 The Real Definition of Anti-Realism

    The meaning of moral anti-realism isn't "morality is subjective." This may appear strange since moral realists think morality is objective. It stands to reason that moral anti-realists think it's the opposite; they are anti-objective-morality. However, there are other ways one can reject the claim that moral facts are true independent of their acceptance from within some point of view, such as claiming that:

1. Moral facts don't exist, moral propositions like "that is wrong" are literally untrue. Our moral judgments are mistaken, so when we judge that it is the case that something is right or wrong, we are literally wrong. This is distinct from thinking morality is subjective since that would be taken to mean that our moral judgments are indeed literally sometimes true.

2. Moral judgments don't aim at the truth. They aren't beliefs or propositions.

So, adding to the anti-objectivist and the relativist^(23) we've been talking about, the agent claiming 1 is known as a moral error theorist while the agent claiming 2 is known as a non-cognitivist.

##3.2 The Amateur's Error in Anti-Realism

    What this entails might take a few steps to break down. While it's similarly a frequent claim by panelists that error theory is an extreme minority position^(24 25), as opposed to non-cognitivism's hold on half of the anti-realists^(22 26), error theory is worth explicating a bit further to make it clear how it compares to the other positions.

The first thing to note here is anti-objectivism, error theory, and non-cognitivism are exclusive.

1. The anti-objectivist and the relativist affirm that there are moral propositions that are sometimes literally true. The error theorist and non-cognitivist denies this.

2. The non-cognitivist denies that moral judgments are beliefs or propositions, claiming that they don't aim at the truth at all. The error theorist, anti-objectivist, and relativist contrarily hold that moral judgments are beliefs or propositions and all claim that moral judgments do, in fact, aim at the truth.

So, anti-objectivists, error theorists, and relativists are all cognitivists. If someone is a non-cognitivist, as half of anti-realists are, they are certainly not any of those other positions, and so error theorists, relativists, and anti-objectivists each make up less than half of a quarter of metaethicists.

One objection I'll be addressing is the objection of positions-close-to-relativism. Often, when it's pointed out that there's simply no room for actual relativism, I've noticed many interlocutors are inclined to point out that they don't literally mean relativism, the position that some propositions are true, but not for everyone. These objectors mean, instead, that some person can have a reason to act on some principle while another person does not have a reason to act on that principle. They might then try to claim that while all the anti-objectivists and moral realists aren't, the non-cognitivists and error theorists are technically on their side.

This will not successfully popularize positions-close-to-relativism. Error theory isn't very popular anyway, but even error theory can resist being a position-close-to-relativism if they take moral propositions to be true without being literally true^(27), which many error theorists have done. Non-cognitivists have even more motivation to not be a position-close-to-relativism since they need to avoid some serious problems they'd face^(18 29) if they couldn't make moral utterances true or false the way beliefs and propositions are true or false; having truth-values means a good reason to avoid being a position-close-to-relativism for the same reason metaethicists think there's a lot of evidence against literal moral relativism in the first place, made undeniably clear by the great deal of ink spilled on the explicit absolutism held by non-cognitivists of every stripe^(30 31 32 33).

##3.3 You're on Your Own, Relativism

    So, I hope the significance of part of this is rather straightforward: Since moral anti-realism doesn't even mean morality is subjective and the other positions are also not moral relativism (or positions-close-to-relativism), all three types of anti-realism can be as popular as they want and moral relativism can still remain fringe. The fact that a fourth of metaethicists defend anti-realism and the fact that moral relativism (and positions-close-to-relativism) are fringe are entirely consistent with one another.

#4. Why should anyone care? How is this important?

    Let's go back to the belief common among non-academics described above that "whatever is moral is arbitrarily decided and is all a matter of mere opinion, making notions like 'evidence' for moral facts and 'arguments' for some thing being wrong nonsensical."

There's a drastic difference in behavior between the person who holds the above belief and the person that holds the belief that morality is not arbitrarily decided and that there are moral standards for which one can find evidence and arguments for. Being the latter, of course, means one should engage with moral discourse in the way one engages with other matters of fact, such as mathematics, epistemology, linguistics, etc.

It should further be noted again that while the consensus against relativism can best be described as very strong, the consensus against moral disengagement because one can just pick whatever moral beliefs they want is unanimous. Among the fringe moral relativists in academia, nobody holds that morality is a matter of mere opinion.

#5. Summary

The reason these are all compatible is:

1. Moral relativism is not the same as morality being subjective, so tons of people can claim morality is subjective and still think it's absolute, making moral relativism unpopular.

2. Morality being subjective isn't even the only type of moral anti-realism. The others are incompatible with moral relativism and there's also a significant amount of evidence against their being like moral relativism.

This is important because:

1. This conclusion means nobody takes the position that 'we should disengage from morality' or 'it's a matter of arbitrary opinion' seriously or thinks it has any compelling evidence.

2. Instead, it's taken to be the case that we should engage with moral evidence just as we engage with scientific and mathematical evidence.

###Endnotes

^1 "[panelist] flair will only be given to those with research expertise in some area of philosophy..."

^2 "...ethical relativism...is an unpopular position."

^3 Wong, David. "Relativism", A Companion to Ethics, 1991, pp. 443.

^4 "relativism is extremely unpopular amongst philosophers."

^5 "It should be said that even these kinds of relativism are at best fringe views among philosophers."

^6 "So, moral relativism must go. It's untenable."

^7 Gowans, Chris. "Moral Relativism", The Stanford Encyclopedia of Philosophy (Spring 2015 Edition), Edward N. Zalta (ed.), 2015, plato.stanford.edu/archives/spr2015/entries/moral-relativism/.

^8 "Moral relativism is an extreme minority position in philosophy, and the version of relativism most popular outside of academic philosophy...is widely recognised as incoherent."

^9 According to metaethicists polled for the PhilPapers Surveys carried out in November 2009.

^10 "...contemporary Kantians take morality to be mind-dependent."

^11 Shafer-Landau, Russ. "Moral Realism: A Defense", Oxford: Oxford University Press, 2003, pp. 15.

^12 Incidentally, these examples (which are taken from Richard Joyce^(13)) also help with the issue of figuring out what these terms mean. It's clear upon reading these examples that the distinction being made between subjectivity and relativity is legitimate, because it makes a lot of sense to us if we say that p is subjectively true even though p is not relative. It's clear that the definitions provided match our usage of these terms as well.

^13 Joyce, Richard, "Moral Anti-Realism", The Stanford Encyclopedia of Philosophy (Winter 2016 Edition), Edward N. Zalta (ed.), 2016, plato.stanford.edu/entries/moral-anti-realism/moral-objectivity-relativism.html.

^ light of recent discoveries of the sibling rivalry between Cameron and Phoenix, much of the contemporary literature confirms this only holds true when not regarding sumo wrestling.

^15 Assuming we're working with Euclidean plane geometry.

^16 Jollimore, Troy, "Impartiality", The Stanford Encyclopedia of Philosophy (Winter 2017 Edition), Edward N. Zalta (ed.), 2017, plato.stanford.edu/entries/impartiality/#IdeObsThe.

^17 Daniels, Norman, "Reflective Equilibrium", The Stanford Encyclopedia of Philosophy (Spring 2018 Edition), Edward N. Zalta (ed.), 2018, plato.stanford.edu/entries/reflective-equilibrium/.

^18 [Weekly Discussion] Enoch's Argument Against Moral Subjectivism

^19 [Weekly Discussion] Explaining moral variation between societies

^20 Gowans, Chris, "Moral Relativism", The Stanford Encyclopedia of Philosophy (Summer 2018 Edition), Edward N. Zalta (ed.), 2018, plato.stanford.edu/entries/moral-relativism/#MetMorRel.

^21 A list of introductory texts for metaethics.

^22 According to moral anti-realists polled for the PhilPapers Surveys carried out in November 2009.

^23 Relativisms aren't all stance-dependent, but including it here can help clarify certain properties of relativism and the other anti-realisms; such as relativism being cognitivist just like anti-objectivism and error theory even if it's incompatible with the latter.

^24 "More common [than nihilism] is error theories of various kinds....This is still an extreme minority view, but at least it isn't simply daft the way nihilism is (most people think it's still pretty daft, though)."

^25 "There is a small minority of ethicists who are error theorists."

^26 "Probably still the most popular version of anti-realism are the various kinds of non-cognitivism, like the sophisticated contemporary versions of expressivism (Gibbard's norm-expressivism; Blackburn's quasi-realism)."

^27 Probably the most emphatically and demanding requested revision was adding a footnote to explain what this means. For reasons^(28) unrelated to the purpose of this specific footnote, I think it's best to explain this by analogy with a closely related position. Some academics working in the field of mathematics and philosophy of mathematics hold that all of our mathematical theories are literally untrue. They reject that objects like " exist, and so their properties fail to exist as well. We would be literally incorrect in saying "7 is prime" or "7+9=16." It seems rather counter-productive to hold such a view and be a mathematician, but of course, our mathematical beliefs are indispensable to our understanding of the world, so we may want to hold that propositions like "7 is prime" are true anyway for some reason other than its literal correspondence to the truth. We may want to hold that moral propositions are true for analogous reasons (e.g. having to do with morality's indispensability to practical reason).

^28 There are several concepts around this topic that, in my experience, can have some difficult baggage when intertwined with morality. For this reason, I often find it far more useful to explain this in the terms of a similar position in some other domain.

^29 van Roojen, Mark, "Moral Cognitivism vs. Non-Cognitivism", The Stanford Encyclopedia of Philosophy (Winter 2016 Edition), Edward N. Zalta (ed.), 2016, plato.stanford.edu/entries/moral-cognitivism/#EmbPro.

^30 Blackburn, Simon. "Is objective^(34) moral justification possible on a quasi-realist foundation?", Inquiry : An Interdisciplinary Journal of Philosophy 42.2, 1999, pp. 213 – 227.

^31 Dreier, James. "Meta‐ethics and the problem of creeping minimalism", Philosophical Perspectives 18.1, 2004, pp. 25.

^32 Blackburn, Simon. "Moral Relativism and Moral Objectivity^(34)", Philosophy and Phenomenological Research 58.1, 1998, pp. 195-198.

^33 van Roojen, Mark, "Moral Cognitivism vs. Non-Cognitivism", The Stanford Encyclopedia of Philosophy (Winter 2016 Edition), Edward N. Zalta (ed.), 2016, plato.stanford.edu/entries/moral-cognitivism/#NonCogRel.

^34 This may seem in conflict with what has been noted prior on this term^(13). There is a good explanation for this: To some extent, it is (it is in the former, but probably not the latter). It should be noted that Blackburn does seem to conceptually distinguish absolutism and objectivism anyway, but it is more important for our purposes to note that taking objectivism to contradict relativism does nothing to contradict the distinction between subjectivity and relativity, both conceptually and in light of plausible metaethical theories.

In the television show Star Trek, they use a device called the transporter to move people around. The transporter scans you, disassembles you, turns your matter into energy, beams you over to another place, and then reassembles you. Very convenient! But one common question about this process is whether you can actually survive it. That is, when the transporter disassembles you, we might think that it kills you, and then when it reassembles "you," it actually just reassembles a clone with all of your memories that thinks it's you. But it's not you: it's a clone. You died! This worry becomes even more pressing in light of episodes of the series in which the transporter malfunctions, and we end up with [two identical people](http://memory-alpha.wikia.com/wiki/Second_Chances_(episode\)). One of them, at least, is a clone. So maybe the transporter is really just a cloning machine and nobody in Star Trek realizes this.

A similar question arises with respect to reincarnation, like for example in Christianity. If, after you die, God is going to bring you back, and he does this by reassembling your body, will he really bring you back? Or will he just create a clone of you?

Finally, some people start to worry that maybe this process happens normally, too. As we age, old cells in our body die and new cells are created. Eventually all or almost all of our cells are replaced. Does this mean that eventually we're just clones with all the memories of the previous person? Do we actually die over time and get replaced by duplicates who are duped into thinking they're us?

In this post we'll learn how philosophers think about this question, and then examine three answers: you don't survive transportation, you do survive, and you maybe don't survive but that's okay.

#How do Philosophers Think About This Question?

Let's use Riker from Star Trek as our example. Let's call him OriginalRiker before he steps into the transporter. The person who steps out the other end is TransportedRiker. Did OriginalRiker survive? If he did, then OriginalRiker = TransportedRiker. They're the same person. In other words, they're identical. If he did not survive, then Original Riker ≠ TransportedRiker. They're not the same person; TransportedRiker is a clone. They're not identical.

So, this question is one about what philosophers call personal identity (see also here, here, and here). Various theories of personal identity give various answers to the transporter question and all the related questions. (There is also a very brief discussion of personal identity in this FAQ post.) We'll see these theories as we look at various answers to the survival question.

#Answer One: You Don't Survive

Some philosophers think that the correct theory of personal identity is a biological theory: we are biological organisms. If OriginalRiker = TransportedRiker then they must be the same biological organism. But of course they are not: OriginalRiker, the biological organism, was destroyed by the transporter, and a brand new one, TransportedRiker, was created at the destination. The new organism is quite similar to the old one, of course - it's an excellent clone - but they're different organisms (one was born decades ago, the other just came into existence) and thus different people.

How does this theory deal with cell replacement over time? Well, are you a new biological organism just because you've replaced a bunch of your cells? That turns into its own separate difficult metaphysical question (sometimes referred to as the Ship of Theseus problem) which you can read about here, here, and here. We can't sort it out right now (it's a topic for another FAQ post, perhaps) but one quick answer is that it looks like our criteria of identity for biological organisms allow for the fact that they change: we don't say a tree is a different tree just because it has grown a lot in 70 years.

#Answer Two: You Do Survive

Other philosophers think that the correct theory of personal identity is a psychological one: we are an instantiation of psychological processes like beliefs, memories, desires, goals, intentions, and relationships. If OriginalRiker = TransportedRiker, then they must have the same psychological properties. And in this case, they do. They have all the same beliefs, memories, desires, goals, intentions, relationships, and so on. So, Riker survives when he goes through the transporter.

Since our psychology survives cell replacement, this means that we also survive over time even though our cells change. But, you might think, our psychology changes too. So, does that mean we die every time our psychology alters? No, because for these theories, personal identity consists not of literally the same psychology but of continuity in psychology: OriginalRiker = X so long as X's psychology is continuous with OriginalRiker's psychology, which means we can trace X's psychology back to OriginalRiker via a series of normal changes. Since the transporter doesn't change your psychology at all, it of course doesn't kill you, nor do other changes, so long as they aren't completely radical. If someone wiped your entire personality, though, this would kill you.

#Answer Three: You Die But it Doesn't Matter

In that episode I referred to earlier, OriginalRiker AND TransportedRiker were walking around at the same time: the transporter didn't manage to destroy OriginalRiker like it usually does. So now the psychological theory is in a bit of a pickle. OriginalRiker can't be the same as TransportedRiker, because they are in separate places doing separate things. But they have the same psychology! Shouldn't they be the same?

Puzzles like this lead some philosophers (most notably Derek Parfit) to suggest that personal identity just consists of psychological continuity when there are no duplicates, which means that when there are duplicates, there is no answer for who OriginalRiker is identical to. So, OriginalRiker, in effect, does not exist any more. So he died. But, it doesn't matter. OriginalRiker's psychology still exists (twice, actually) and that's what matters. So if we want, we could even say that you die every time you step into the transporter. It doesn't matter! What does matter is that your psychology continues to exist. So who cares if the transporter kills you? Survival is not what matters. Psychological continuity matters. If you can secure psychological continuity for yourself, then you're good to go. In the normal world, the only way to secure it is via survival. But if we invent transporters, you could also secure it by making sure the transporter will rematerialize you (or, rather, your clone) after it destroys you.

#Conclusion

This has not been an exhaustive look at the topic. There is much more to say. For instance, one other theory of personal identity is the "soul" theory: you are you because you have you soul. If this is the right theory, then the question is whether your soul can find your new body once you step into the transporter. For what it's worth, according to this survey, approximately 36% of philosophers think you would survive a transporter and 31% think you'd die. About 33% of philosophers endorse the psychological theory of personal identity, while 17% endorse the biological view, and (very roughly) about 12% endorse something like the soul view.

#More Resources

In addition to the articles linked above, which are quite comprehensive, Derek Parfit's book Reasons and Persons has some great discussion of these topics.

Here are some YouTube channels that have good philosophy content. If a YouTube philosophy channel is popular but not listed here, chances are it's bad. Crash Course Philosophy, for instance, is bad. So is 8-Bit Philosophy. Jordan Peterson and PragerU are very bad.

• Wireless Philosophy
• Brain in a Vat
• Philosophy Overdose
• Agency and Responsibility Research Group
• Dr. Greg Sadler
• Overthink Podcast
• Simon Kushing
• Brains Blog
• Friction Philosophy
• International Social Ontology Society
• Monte Johnson
• Wes Cecil
• Kane B
• European Graduate School Video Lectures
• flame0430
• The Funky Academic
• An Introduction to the History and Philosophy of Science
• Jeffrey Kaplan
• Thoughtology
• Carneades.org
• Rotman Institute of Philosophy
• Sean Carroll
• The Royal Institute of Philosophy
• Ian Ramsey Centre
• Kant's Critique of Pure Reason by Robert Paul Wolff
• The Great Philosophers: A History of Western Philosophy
• ContraPoints
• Philosophical Techne
• The Analytic Tradition

This is a long one, ladies and gentlemen. The tl;dr is that time travel to the past can't be ruled out on a priori grounds, and might be permitted by the laws of physics, too: that remains an open empirical question. There's a lot of detail to all of this, though, so read on!

Fair warning: here there be physics.

We'll start with the basics to get a general idea of what's going on here. Next, we'll look at a simple time travel story to use as a case-study. Then we'll discuss the grandfather paradox and the trouble with changing the past. From there, we'll get a conceptual overview of the physics of space and time in general relativity, before finally closing with a discussion of the relationship between teleportation and time travel.

The Basics

Whether or not time travel to the past is physically possible is an open empirical question. There are solutions to Einstein's field equations of general relativity that permit closed timelike curves (which is what would be physically necessary for time travel to your past), but there are also solutions which don't permit CTCs. Which kind of universe we live in is still an open question.

That said, there's nothing logically incoherent about time travel to the past, and we can't rule out the possibility on purely a priori grounds. The most important things to remember when thinking about time travel to the past are (1) that everything only happens once and (2) history must be consistent. If you can keep that in mind, these problems get easier to think about. It's true that time travel to the past can result in you being "bilocated"--that is, occupying two distinct spatial locations at the same time--which can get confusing to talk about, but ultimately isn't a serious problem. In fact, any time you travel to a past year in which you were already alive, you'll always be bilocated for the period of time between the two "ends" of the time machine: the bilocation starts at the moment at which the "past" end of the "time portal" opens, and ends at the moment when the "future" end of the "time portal" closes.

A Simple Time Travel Story

This all gets a lot easier to think about if we track different individuals continuously. Let's narrate a simple time travel story in chronological order. We'll call our time traveler "Bob." Let's say Bob is 30 in 2016.

2011

Here's Bob. Nothing much happens this year, except Bob starts thinking about time travel. Bob is 25.

2012

Bob is going about his business, and he's 26 this year. One day, a portal opens in his bedroom, and out steps a slightly older version of himself. This guy (we'll call him "Robert" so we can keep them straight) is 30 years old, and he came from 2016. Robert says "I know you're kind of freaked out, because I remember how freaky this was. Don't worry about it, though: we're cool. Also, time travel feels weird, as you'll find out in a few years." Bob and Robert high five each other. Bob/Robert bilocation starts at the instant Robert steps out of the portal.

2013-2015

Bob and Robert go about their businesses. They look very similar--like identical twins--but they're distinct entities. Robert however, remembers doing everything Bob does during this time, since he already lived through it. Robert helps Bob build his time machine.

2016

The time machine is done! Bob is now 30 and Robert is 35, though his birth certificate says he was born in 1986. He's older than he "should" be because he lived through 2011-2016 twice. Bob and Robert fire up the machine, and set it for 2011. "I'd tell you to have a good trip, but I already know you do" says Robert. Bob hops into the portal, and it closes. Robert carries on with the rest of his life. Bob/Robert bilocation ends at the instant Bob enters the time machine.

Between 2012 and 2016, Bob and Robert both exist. It's tempting to think that there will now be three versions of Bob in 2012, but that's not right because everything only happens once. The Bob who gets into the portal in 2016 steps out of the portal in 2012 as Robert. He remembers interacting with the time traveler in his past, but now he is the time traveler. When he arrives in 2012, he finds the 2012 "version" of himself waiting for him, and everything plays out just like it does in the story.

Fin.

"But wait," you say, "what if Bob decides to do something different this time when he goes back?" We already know he doesn't! His "doing something different" is logically incoherent in just the same way that a statement like "yesterday, I both did and didn't take a shower" would be. Whatever it is that he intends to do, we know what he in fact ends up doing: whatever he recalls the time traveler doing (assuming his memory is accurate). This is part of the "everything only happens once" dictum. It's not the case that when he goes back in time in 2016 he gets a "do over" and a chance to change what happened the "first time:" that way of thinking implicitly assumes that this is the "second time" he's going back, or that he gets a chance to consider his actions again. But that's not right--the time travel event happens once and once only, and Bob does whatever he does. If (counterfactually) he had done something different, then he would remember it that way. There's no opportunity to "revise" events, because there's no "second time around." In time travel metaphysics literature, this is sometimes called "the second time around fallacy." We know that no matter what, things turn out the way they turned out.

The Grandfather Paradox and The Trouble With Changing the Past

OK, suppose that Bob decides he's going to change the past (maybe even to kill his past self), and gets in the time machine with that intention. What's stopping him from doing that? There's no general answer to that question (other than "something that worked")--there is, as far as we know, no "time police" or supernatural being that's responsible for maintaining consistency. Rather, we just know that even if Bob gets into the time machine with a sincere intention to (say) kill his grandfather, things somehow work out in such a way that he does not succeed and everything happens the way it did. Bob might have a last minute change of heart, might decide it's a bad idea, might miss his chance, his gun might misfire, or a million different other things could happen, but the notion of changing the past is logically incoherent. He can affect the past in the same way that anyone present at a particular time contributes causally to what happens at that time, but one (and only one) sequence of events happens.

This still seems vaguely unsatisfactory. Intuitively, we want to think that if Bob remembers what Robert did during the time that he was present in the past, then when Bob steps into the time machine, all he has to do is behave in a way that's different from what he remembers, and he'll have changed the past. Stipulating that he can't (or doesn't) behave in a way that's different seems to suggest either that there's no free will at all, or (equivalently, I suppose) that the universe has to "conspire" to force him to behave in the way he remembers Robert behaving, possibly via a strong of wildly improbable coincidences. That's the intuitive pull of the Grandfather Paradox: if time travel is possible and I really want to kill my grandfather, then what's stopping me? It seems like there would have to be a string of astronomically improbable coincidences to keep me from doing something like that.

That's true, I suppose. On the other hand, it's also true that the number of coincidences that have to line up just right in order to make things turn out the way they do even without time travel are similarly improbable. We might worry that this line of argument against the possibility of time travel is uncomfortably similar to arguments from incredulity against evolution by natural selection as well. People critical of evolutionary theory make similar observations about the extreme improbability of all the coincidences necessary to result in a complex organism like a human being developing through "random mutation" and natural selection. That argument, of course, is a mistake--even though the evolutionary history of any particular organism is highly improbable (at least in a certain sense). Indeed, the causal history of any system represents the accumulation of a tremendous number of what the physicist Murray Gell-Mann famously called "frozen accidents:" small contingent events that ended up playing a pivotal causal role in the eventual outcome of some process, the precise sequence and causal history of which was extremely unlikely to have turned out way it did. From a certain perspective, every event is the result of a staggeringly improbable sequence of coincidences, given all the other things that might have occurred which would have led to a different outcome. Given a uniform distribution over all possible causal histories, the odds of all those things lining up just right for this state of affairs are astronomically small.

And yet we accept that string of coincidences without blinking; from another perspective, the probability of the precise causal history of the universe up to this point having happened is 1, because it did happen, and the post-hoc probability of any actual outcome is 1. From that perspective, why should we think that the string of coincidences a time traveler experiences strains credulity any more than those in the normal course of events? The answer almost certainly has something to do with the fact that such a sequence of events would imply uncomfortable things about our own agency in the world, and raise (perhaps unwelcome) questions about how we should understand our contributions to the causal history of the universe. It's not clear to me that this is a fatal objection to the possibility of time travel, though--radical discoveries in physics (and science broadly) often do great violence to our intuitions about how things work, and about our place in the natural world. The usual response to that happening is "well, so much the worse for our intuitions then." If science has taught us anything, it's that the natural world is deeply strange in lots of ways, and it isn't obvious that this would make it significantly stranger than (say) general relativity or quantum mechanics.

This might seem really weird, and it definitely is. Time travel to the past breaks a lot of our intuitions, and causal histories in the vicinity of time machines are likely to be somewhat bizarre. In some cases, someone with a sincere intention to "change the past" might be foiled by an astronomically improbable string of coincidences. In other cases, an attempt to change the past might end up bringing about the sequence of events the time traveler was hoping to foil (this is the central plot device of the film 12 Monkeys, for instance). The region around a time traveler or time machine is likely to be very casually strange, but history is necessarily consistent, and happens only once. As long as we avoid the second time around fallacy, there's nothing logically incoherent about time travel to the past.

The Physics of Time and Space

Let's talk a little bit about what space and time are (and how they relate to each other) from the perspective of contemporary science. In general relativity, both space and time are part of a common manifold, so each of them is a measure of one component of the distance between two points (or events). You can think of it as being somewhat analogous to more familiar vector addition in Euclidean space. If I walk three miles west, and then four miles north, my actual distance from my starting point is (by the Pythagorean theorem) five miles, despite the fact that I've walked seven miles total. I can isolate that distance into a "northern component" and a "western component," though, and just talk about my distance in either of those two directions. Distance in spacetime works the same way, only I can separate the distance between two events into "spatial" and "temporal" components.

In general relativity, there are three different sorts of "intervals" by which two points on the pseudo-Riemannian manifold of spacetime can be separated: space-like intervals, time-like intervals, and light-like intervals. The precise formulation of all of these isn't all that important here (you can find some of the basic details for the special case of a locally flat Minkowskian spacetime here if you're curious), but the general idea is that the different intervals reflect the various ways in which two points can be separated from one another on a four-dimensional manifold. Light-like intervals are uniquely interesting because for any light-like interval of a given length, the value of the spatial component of the distance is exactly equal to the value of the temporal component of the distance: two points that are light-like separated are equally far apart in space and time, and so the general spacetime interval between them is said to be zero.

You can think of the light-like interval between two points as the maximum total distance with which the points could be separated such that information could pass between them. This has deep implications for the geometry of spacetime, since light-like intervals define the light cone for an event, which (because it is Lorentz invariant) gives spacetime an element of structure upon which all observers will agree, irrespective of their relative motion to one another. Because all observers agree on the speed of light, light always takes the shortest available path between two points in spacetime.

That is, a light-like interval is analogous to a "straight line" through Euclidean space: it defines the most efficient way to cover the total distance between two points. In higher dimensional spaces like this, such paths are called "geodesics." It's impossible to go faster than the speed of light in just the same sense that it's impossible to find a shorter distance between two points on a piece of paper than a straight line connecting them. The reason time "slows down" as you accelerate is that the curve of your path through the temporal dimension is "flattening" out as it approaches a light-like path, and so the temporal component of your path becomes shorter and shorter--you're actually moving through "less" time to cover the same spatial distance. This is analogous to aligning your path through Euclidean space more and more with a straight line between two points, rather than choosing a more roundabout route. The length of the light-like path between two points tells you the minimal amount of time you have to cross in order to get from where you are to a particular point in space, in just the same sense that the length of a straight line connecting two points on a sheet of paper tells you the minimum distance you have to traverse to move between those points.

The light cone structure also defines the future and the past in the context of relativity. Rather than being absolute ideas, "future" and "past" are associated with particular events or points in space-time. If we pick a point in spacetime (that is, pick a spatial location and a specific time), we can imagine an observer at that location activating a flashbulb or causing an explosion, creating a pulse of light. This pulse of light expands outward from the original point in an ever-increasing sphere of light (on a light-like trajectory). As we move toward the future, the sphere gets larger and larger, encompassing more space at every instant. In space, the propagating light looks like a series of larger and larger spheres, but if we "stack up" all those spheres to form not a space but spacetime, we get a four-dimensional cone of expanding light, with the "tip" of the cone sitting on the original location from which the pulse of light originated, with the cone "growing" as we move toward the future.

This cone--the future light cone--specifies a collection of spatial points at every time that a pulse of light could reach if it had been emitted at our original location. These are the points that an observer standing at the original point could potentially communicate with by sending a pulse of light (or anything else), and thus could potentially causally interact with. This collection of points constitutes the future light cone for the original spacetime point, and represent all the points that lie in the potential objective future of an observer at that point: they're points that the observer could potentially travel to by moving at a speed below the speed of light, and thus events that the observer could potentially influence (at least in principle). This is what "the future" means in the context of relativity.

Going in the other direction, we can imagine a similar cone pointed the other way: a cone with its "tip" sitting on the same original spacetime point and "growing" toward the past. The collection of spacetime points inside this cone represent all the spacetime points that could potentially have influenced this event by sending a pulse of light (or anything else) to this location. The further back in time we go, the more spatial distance a pulse of light could have covered, and thus the more events could have (in principle) causally interacted with our observer. This is what "the past" means in the context of relativity: all of the events that could possibly have had some influence on you, or (equivalently) all of the spacetime points from which something could have traveled to reach you by moving at a speed less than that of light.

Points that lie inside an event's past or future light cone are timelike separated from that event. Since all observers agree on the speed of light, all observers agree on this light cone structure. This means that the events that lie in a spacetime point's future and past are Lorentz invariant, and thus agreed upon by all observers. However, since "the past" and "the future" are keyed to this light cone structure, it's possible for precisely the same point to lie in the future of one event, and the past of another. Though all observers agree on the past and the future of any particular spacetime point, what counts as "the future" and "the past" varies from point to point: what's in your future or your past depends on where you're standing.

Points that lie in neither the future nor past light cone of an event are said to be spacelike separated from that event. Two events that are spacelike separated can have no causal influence on one another, since information cannot pass between them. Strange as it seems, an event that is spacelike separated from you is neither in your future nor your past. General relativity is super weird.

But what if you could move between different points of space? What if you could teleport? Strange as it seems, this would allow you to time travel also--at least in a sense The deeply connected nature of space and time in general relativity means that even without moving in the timelike direction at all, teleportation would give you access to the past.

Teleportation and Time Travel

We can leverage all this to see why if you could teleport--instantaneously move between spatial locations--you'd actually be able to time travel as well. Let's set this up carefully to see why the existence of the Lorentz transformation is compatible with there being no unique past or future, and why that allows for a certain kind of time travel. To begin with, remember that when we talk about "events" in the context of general relativity, what we're really doing is locating a particular point (or region) in spacetime. That is, we're picking out a specific location on the four-dimensional manifold representing both spatial location and temporal ordering. This is just like picking out a particular location on a sheet of paper by specifying its Cartesian coordinates; while the paper is two-dimensional (and so every point can be uniquely specified by giving two numbers), spacetime is four-dimensional (and so every point can be specified by giving four numbers).

Just like on a sheet of paper, there are an infinite number of ways that we might go about assigning coordinates to points in spacetime: we can do things like rotate our axes, move the origin around, and so on. In any given reference frame, an observer can use his own local coordinate system--usually specified through reference to something like a ruler for space and a clock for time--to locate and order events from his perspective. To transform one observer's coordinate system to another observer's coordinate system, telling us what some set of measurements made in the first frame will look like to an observer in the second frame, we use a mathematical procedure called the Lorentz transformation.

Some quantities are changed in the transition between any two coordinate systems--reference frames--and some aren't. One of the most important unchanging quantities is the speed of light: no matter which frames we choose, all observers will agree on the speed of light. Quantities which are conserved (like the speed of light) are said to be Lorentz invariant. This has some odd implications, as for the speed of light to be Lorentz invariant, lots of other quantities must fail to be. Things like duration and length are altered by the transformation, leading to phenomena like time dilation and the relativity of simultaneity.

This is easiest to see with a thought experiment (this formulation was Einstein's). Suppose that you're on a moving train and I'm standing on the platform as you pass by. Because of the train's velocity, you're moving at some speed (say, 100 mph) relative to me. Suppose that, while standing in the middle of the car, you light a match. Because you're at rest relative to the train car, the light from the match (which moves out from the match in an expanding sphere) has to cover the same distance to reach both the front and back of the car, so you see the light hitting both walls at the same time.

From my perspective, though, the walls of the train are moving, so the back of the train is "catching up" to the light and the front of the train is "running away" from the light. However, we both agree on the speed of light--the velocity of the train car doesn't add or subtract to the speed of light. The only way for these two facts to coexist is if I see the light hit the back wall before it hits the front wall.

So you and I disagree about the ordering of some events: you think the light hit both walls at the same time, and I think it hit one wall first and another wall later. If you think about this for a moment, you'll see that it implies that "future" and "past" have different meanings for each of us. There is, after all, some moment (for example, the moment just after each of us sees the light hit the back wall) at which the light's hitting the front of the train has already happened for you--and so is in your past--and has yet to happen for me--and so is in my future!

So why does this mean that teleportation allows for time travel? Suppose Alice is on the train car and Bob is on the platform. Suppose there's also another observer (Charlie) standing at the front of the train car. When Charlie sees Alice's flash reach him, he teleports to the train platform. But Charlie will see the flash reach him at the same time that Alice does, since they're at rest relative to one another and at rest relative to the train, while the light has further to go from Bob's perspective. Therefore, the light triggers Charlie's teleporter and he appears next to Bob before Bob sees the teleporter trigger: he's come from Bob's future--which is also his and Alice's past--without moving through time at all. He can't get to his own past (or future), though, because by hypothesis he can only reach events that are spacelike separated from his starting position, which means that he can only get to points that are in neither his past or his future.

Since he arrives on the platform (and then watches himself teleport away), it seems like he's arrived in his own past (after all, he both remembers himself teleport and watches it happen), but that's not quite right. Remember, "past" and "future" aren't associated with observers, but rather events or spacetime points. When Charlie teleports around, he doesn't carry his past and future with him, but rather is stuck with the past and future of wherever he ends up. The trick here is that "the flash hitting the front of the train" doesn't specify a unique event. Rather, the relevant events are "Charlie sees the flash hit the front of the train" and "Bob sees the flash hit the front of the train" (this is a consequence of space and time being part of the same manifold), and so things that are simultaneous with one need not be simultaneous with the other one: it all depends on how you're moving. The time travel is possible here because by teleporting, Charlie is moving instantaneously between both spatial locations and reference frames. By switching locations, he changes which set of light cones correspond to his future and past. By switching reference frames, he changes which events he judges to be simultaneous. By cleverly manipulating those two facts, he can effectively time travel--but only from the perspective of where he ends up.

It's very important to remember that the differences in ordering here aren't just apparent: they're of real, objective physical significance. The light really does hit the front wall at the same time it hits the back wall for those at rest relative to the train, and the light really does hit the back wall first for those moving relative to the train.

The formal way to think about this stuff is by taking what amounts to a three-dimensional "cross section" of spacetime and seeing what's in it. If you can imagine this sequence of events as consisting of a long sequence of "snapshots" of what's happening everywhere in space and then strung together, that's the right picture. We're cutting a three-dimensional slice out of four-dimensional spacetime, with the "cut" happening along the time dimension. In physics, they call that slice a "timelike hypersurface." All events that lie along the timelike hypersurface are simultaneous.

The issue is that because elapsed time is not Lorentz invariant, exactly what events do and don't lie along a timelike hypersurface depend on your frame. For Alice, the light hitting the front and the back lie along the same timelike hypersurface. For Bob, the timelike hypersurface that intersects the light hitting the back wall and the timelike hypersurface that intersects the light hitting the front wall are distinct. So when Charlie teleports from the train to the platform, the succession of timeslices (we call a particular way of slicing things a "foliation of spacetime") he's working with changes. Things that were simultaneous from his old perspective no longer are, and some of what was in his past light cone is now in his future light cone. This is not just an apparent change: it's a real one.

This sort of time travel isn't what we're usually talking about when we talk about time travel. In this case, we've got instantaneous movement across spacelike separation. In "regular" time travel, we're talking about the creation of closed timelike curves, which are physically and topologically distinct kinds of things with distinct effects. CTCs do allow you to traverse your own "starting" past light cone, for one thing. If your spacetime path is a CTC, you're effectively "tilting" your light cone onto its side. (As an aside, all acceleration involves a kind of "angle change" in your light cone, which is one way to think about both time dilation effects and what a black hole does; spacetime beyond the event horizon of a black hole is so warped that it distorts your future light cone such that your entire future lies inside the event horizon. This is why acceleration and gravitational attraction are equivalent in GR.) With a CTC, the "tilt" is so pronounced that your "future" includes both events that are spacelike separated from you and events that are part of your original past light cone.

Further Readings

For a somewhat more detailed discussion of time travel, David Lewis' ["The Paradoxes of Time Travel"](http://www.csus.edu/indiv/m/merlinos/paradoxes of time travel.pdf) is a good read. In fiction, the short story "By His Own Bootstraps" by Robert Heinlein a great example of a consistent, coherent, and interesting time travel story. The SEP on time travel is also a good reference.

Sometimes people wonder why they are themselves, rather than someone else. Why do I see the world from my eyes, rather than yours? How come I am thinking my own thoughts, rather than yours? This seems like a pretty deep question but the answer is pretty trivial. It does, however, bring up an interesting topic in philosophy, which we will address once we look at the answer.

#You are you because you aren't someone else

The trivial answer to these questions is that you see the world through your eyes, think your own thoughts, and inhabit your own body because words like "your" and "your own" just by definition refer to your eyes/thoughts/body rather than someone else's. If you saw the world through someone else's eyes, those eyes would be your eyes, rather than the ones you have right now.

In other words, you see the world through your eyes because the thing that you are has these eyes rather than any other eyes. You think your own thoughts because the thing that you are thinks these thoughts rather than any other thoughts. You inhabit this body because the thing that you are has this body (or is this body) rather than any other body.

Why are those things true? Well, for very boring reasons. You have your body because your parents had sex and a fertilized egg developed for a while into your body. You have these eyes because as part of that development process, some cells in the fetus differentiated into eyeballs. You think these thoughts because your mind developed in a certain way via education, observation of the world, and so on.

However, this does bring us to an interesting question. If you have your eyes because the thing that you are has these eyes, and you think your thoughts because the thing that you are thinks these thoughts, and you have this body because the thing that you are is in this body, what is the thing that you are? What are you?

#What is this thing that I am?

This is known in philosophy as the question of personal identity (see also here). Various philosophers give various answers.

One very simple answer that almost no philosopher endorses is that you are a soul, whatever that might be. Then the answer to questions like "why am I me?" is "because each soul is one person, and other people have other souls," and the answer to "why do I see the world through these eyes?" is because your soul is linked to these eyes (perhaps because God hooks souls up to bodies for us).

Two more popular answers are that you are your thoughts, or that you are your body.

The first sort of theory is known as a psychological theory of personal identity. According to this theory, if you could transfer your thoughts to another brain and body, or even a machine, you would be in a new body, or in a machine. Then you'd see the world through new eyes, or even no eyes.

The second sort of theory is often known as the biological theory of personal identity because on this theory you are a biological organism. Thus you see the world through these eyes because these are the eyes that you (a biological organism) have and you think these thoughts because these are the thoughts occurring in your (a biological organism's) brain.

There are other theories of personal identity too. Those are just a few options.

#Could I have been born a different person?

Now we can answer the final question in the title. Could you have been someone else? If we're asking whether you could have been born into a different body, the soul theory says "for sure!" Your soul could have gone into my body when I was born, for instance. Then you would have been me, rather than you.

The other two theories of personal identity we've discussed would say no, you couldn't have been born someone else. For the psychological theory, it's technically possible that your thoughts would have come about in someone else's body, but that's so ridiculously improbable (because your thoughts include things like attachment to your specific parents and friends, and thus you'd have to be born into another body but still end up attached to these parents and friends in the same way) it's effectively impossible. For the biological theory it's for sure impossible - one biological organism can't be another biological organism.

In effect, you can imagine asking of an apple, "could this apple have been another fruit, like for instance an orange?" If you think fruits have souls, then we could imagine the apple soul, instead of being placed into an apple seed, being placed into an orange seed, and growing from that point on. In that case, the answer is "yes, this apple could have been an orange." Since fruits likely don't have souls, though, a better answer is "no" - "this apple" just refers to the set of characteristics that make a fruit an apple, and if those characteristics didn't exist, this apple wouldn't exist. Since those characteristics don't exist in an orange, there's no way this apple could have been an orange.

#Further Reading

https://www.reddit.com/r/askphilosophy/comments/2jxso7/why_am_i_me/

https://www.reddit.com/r/askphilosophy/comments/1yrmfr/why_am_i_me_and_not_you/

https://www.reddit.com/r/askphilosophy/comments/2kt2an/why_are_we_who_we_are/

https://www.reddit.com/r/askphilosophy/comments/2bxaz2/what_is_the_thing_that_is_experiencing/

https://www.reddit.com/r/askphilosophy/comments/33eobc/the_thing_about_consciousness_i_just_cant_wrap_my/

https://www.reddit.com/r/askphilosophy/comments/25axjj/why_am_i_conscious_in_this_body/

https://www.reddit.com/r/askphilosophy/comments/6icqme/why_am_i_me_and_not_you/

TL;DR - Falsificationism is a proposed way of distinguishing science from pseudoscience, and understanding what's distinctive about scientific theories. It suggests that scientific theories are scientific in virtue of there being some test that could prove them incorrect. If a hypothesis conflicts with what an experiment shows, according to this theory, then it should be abandoned. Strict falsificationism isn't widely accepted anymore today, for reasons outlined below.

It's much harder to collect evidence demonstrating the truth of a general positive assertion than it is to collect evidence demonstrating the falsity of a similar assertion. Strictly speaking, to know that a statement of the form "the speed of light through some medium is an absolute upper limit on the speed of anything through that medium," we would need to look at the motion of every possible thing through every possible medium in every possible situation. That's not practical, but only a single reliable instance of something moving faster than light through a medium would serve to demonstrate that the proposition is false.

That's why Karl Popper put such great stock in falsifiability. Intuitively, it seems like what makes science distinctive when compared to other approaches to understanding the world is that science makes claims that can be tested, and abandons claims that fail to stand up to experiments. If we devise an experiment that would succeed if some hypothesis were true and the experiment fails, then we know the hypothesis was false and move on. If you ask an average person on the street to define science, they'll probably say something about falsifiability.

Of course, in practice things are rarely that simple, and even falsification is almost never a straightforward task. Very few philosophers (and virtually no scientists) think that strict falsificationism is a good way of understanding what science does these days. The Quine-Duhem thesis (QDT) is a pretty serious challenge to the Popperian falsifiability criterion for demarcating science from non-science, and it's generally accepted now that simple falsifiability isn't naunced enough to capture what's distinctive about scientific reasoning.

The main thrust of the QDT is that scientific hypotheses/theories are extremely difficult to isolate in a way that would make them amenable to direct falsification. When we run a particular experiment, we're not only testing the hypothesis itself, but also a large number of interrelated background assumptions or "auxiliary hypotheses." A negative result on such an experiment might be explained by the fact that the central hypothesis is false, or it might be explained by some error in one of those auxiliary hypotheses.

The best contemporary illustration of this that I can think of is the big hooplah about superluminal neutrinos that happened five or six years ago in theoretical physics. The OPERA lab at CERN reported that they'd discovered some neutrinos (very, very light but not-quite-massless elementary particles) traveling faster than light in their experiments, a result that would seem to straightforwardly falsify some pretty major things (like special relativity, for instance). Science journalists lost their fucking minds about the result, despite the fact that the scientists involved were extremely cautious in their statements about what the results meant. Headlines screamed about SR being overturned, while the scientists were jumping up and down yelling that we shouldn't get too hasty. Internal retests managed to replicate the results, and some outside retests also seemed to find consistent results.

On closer inspection, it turned out that the detector apparatus at OPERA...had a loose cable. One of the fiber optic cables responsible for measuring the travel time in the experiments was slightly unplugged, which generated anomalous results. Once they fixed that connection, the anomaly disappeared and everything went back to normal.

The big lesson to draw from this is that in practice scientists generally don't hold to anything so strict as Popperian falsifiability. If they had, the experimental results (especially after they were reproduced) would have caused us to abandon special relativity, since it had (apparently) been falsified. This didn't happen, which in retrospect was a very good thing, since the experiment didn't actually show what we might have taken it to. Rather than just testing the hypothesis "massive particles can't go faster than light," the experiment was also testing things like "all of our tools are in good working order," "our experiment is set up right," and "our measurement devices are sensitive enough to detect the relevant quantities." It turned out that one of those hypotheses was false, not the more substantial main hypothesis.

This is pretty characteristic of scientific experiments, and disentangling the main hypothesis from all the auxiliary ones is often extremely difficult (if not impossible). Another good illustration was the famous attempt to detect gravitational lensing during an eclipse during the early part of the 20th century. This was a very well-understood phenomenon that was predicted by general relativity, and failing to find evidence for it would have straightforwardly falsified GR. The earliest experiments failed to detect any lensing, but nobody seriously considered abandoning GR. Rather, they assumed that their instruments were simply not sensitive enough to detect the effect at that point, and went about designing better tools. This turned out to be correct, and the effect was detected a few decades later. Deciding when a hypothesis has been compellingly falsified (and when the hypothesis should be maintained even in the face of experimental evidence) is very much a judgement call, and not something that can be used as a strict arbiter of the success of a theory. Things are more complicated than that, as usual.

See also: Underdetermination in science, Pierre Duhem, scientific progress, scientific explanation.

What is functionalism? What do functionalists believe? Is this the same thing as computationalism about the mind? Is functionalism a "physicalist" theory?

TL;DR - Functionalism asserts that "having a mind" is just a matter of having the right kind of relationship between inputs and outputs. Anything that plays the right kind of "functional role" in a larger system--anything that coordinates inputs and outputs in the right way--will count as a mental state, irrespective of the material it's realized in.

My go-to example for illustrating functionalist thinking is the notion of an adding machine. Adding machines can take on a lot of different physical forms: from the old-fashioned adding machines with levers and rolls of paper tape, to a TI-86 graphing calculator, to a program on your iphone, to an abacus, to this super awesome contraption of wooden levers and ball bearings (seriously, go watch that video: it rules). What makes all these things instances of adding machines is that their internal structure is designed in such a way that they're able to take certain inputs, transform those inputs in regular ways, and produce certain outputs. Whether those inputs take the form of the initial position of metal balls, the position of beads on a wire, voltage states on a silicon circuit, or whatever doesn't matter: they're all (partially) isomorphic to one another in a way that lets them work as adding machines. The wooden stops and balls in that video are functionally identical to certain patterns on a circuit board combined with certain rules for shifting electrons around.

When I teach philosophy of mind, I like to frame functionalism as a synthesis of mind/brain identity theory and logical behaviorism--it takes a lot of what's good in both those positions, and discards a lot of what's bad in both of them as well. It's helpful to understand a little bit about what each of these positions asserts, and how they both tie together in functionalism.

Mind/brain identity asserts that mental events are (in some sense) identical to brain events--your mind just is your brain, and your thoughts just are particular patterns of neurons firing. This is right in a lot of ways; minds are part of the physical world, and it doesn't make sense to think about a mental event that isn't associated with some physical system.

Logical behaviorism, on the other hand, asserts that when we talk about mental events, we're really just talking about behavioral outputs. Pain, on this view, corresponds to a particular constellation of behavioral dispositions (to avoid some stimulus, to report that the application of that stimulus hurts, to flinch, &c.). On this view, the only thing that matters is the output: the processes that generate the output are irrelevant, as are the physical systems in which those processes are instantiated. This also is right in some important ways.

Both mind/brain identity (MBI) and logical behaviorism (LB) also get a lot wrong, though. MBI, on most formulations, is far too rigid; strictly speaking, it implies that no system could have mental states like ours unless that system had something that was just like our brains. MBI has a hard time dealing with multiple realizability, because it's stuck on a fairly strong identity claim, and overly focused on the specific organizational details of the brain. It's hard to understand how something organized very differently might also have mentality, on this view. LB has the opposite problem. By ignoring the details of the system that produces certain behaviors, it's overly fixated on the outputs of some process; it suggests that facts about how various outputs are produced are irrelevant, and that any system that gives the right output at the right time has mentality (while any system that doesn't give the right output doesn't have mentality). These mistakes are, in some sense, complementary to one another: MBI is too focused on the minute details of the organizational structure of the brain, and LB is too focused on the behavior outputs of the brain.

Functionalism represents a nice middle-ground between these two views. For a functionalist, having mentality consists in not just giving the right outputs in response to some inputs, but generating those outputs in the right way. That is, functionalism focuses on whether or not the physical processes that produce some behavior in response to some stimulus are relevantly similar to the physical processes that take place in the brain under the same circumstances. If they are, the functionalist is willing to say that the system has mentality. That is, if some pattern in the activation of digital circuits (or chambers of gas, or interaction between insects, or whatever) plays the same role in the overall coordination and behavior of some system that a particular pattern of neuronal activity does in us, then the fact that digital circuits (or whatever) are being used instead of neurons doesn't matter. If some piece of programming plays the same role as whatever's going on in my head when I have a belief does, then that piece of programming just is a belief. Think again of the case of the adding machine. The MBI-analogue theory would assert that the process of adding is identical to the operation of some physical device with a roll of tape and a level. The LB-analogue would assert that all there is to "adding" is spitting out the right numbers at the right time. The functionalist view asserts that "adding" consists in spitting out the right numbers at the right time, and doing so because of some set of reliable physical operations that are all relevantly similar to one another. This position has many of the strengths of mind/brain identity (e.g. the observation that what's "under the hood" so to speak matters in these discussions), as well as many of the strengths of logical behaviorism (e.g. that the production of behaviors is a central aspect of mental states), but it both allows for multiple realizability and attends to the process by which behaviors are generated.

This isn't the way functionalism is usually presented, but I think looking at it dialectically like this makes the motivation for embracing functionalism much more clear, especially when you consider its place in the evolutionary history of philosophy of mind. There are, of course, still problems with the functionalist account of the mind, but it represents significant progress over both logical behaviorism and mind/brain identity, and can be reasonably thought of as the synthesis of those views.

See also: the SEP entries on functionalism, the Turing Test, the computatational theory of mind, and mental representation. Ned Block's article "The Troubles With Functionalism" is a good introduction to some of the major objections, and David Lewis' "An Argument for the Identity Theory" further elaborates on the connection between mind/brain identity and functionalism.

As pointed out elsewhere in this FAQ, philosophy is a very large field. You could spend your entire life studying political philosophy and never touch metaphysics, or vice versa. This makes it difficult to recommend good places to start for people who know nothing about philosophy: it's a bit like recommending a food to someone who has never eaten anything. Do you recommend a peanut butter and jelly sandwich? A South Indian dal? Roasted Brussels sprouts? Tacos?

This FAQ answer, therefore, will focus on two main things. First, it will list some general overviews that will help you get familiar with various parts of philosophy and help you figure out where your interests lie, if they do in fact lie anywhere in philosophy. Second, it will list some good starting points for various topics that newcomers are commonly interested in.

#Good General Overviews

This section divides into two: short, broad overviews, and more extensive histories.

##Short, Broad Overviews

Two good places to start are Simon Blackburn's book Think and Julian Baggini's The Pig That Wants to be Eaten. The Blackburn book clearly and concisely goes through most of the large areas of philosophy and talks about how various philosophers have thought about them. The Baggini book presents interesting little thought experiments and puzzles that can help acquaint you with some of the interesting and odd topics that philosophers have addressed. Both books provide enough citations for you to start digging on your own once you've identified things you find interesting.

Four other broad starting points are Kwame Anthony Appiah's Thinking It Through, Thomas Nagel's book What Does It All Mean?, Bertrand Russell's book The Problems of Philosophy, and this collection of five dialogues written by Plato (Plato's dialogues can also be found free online, although the translations are not always great). The Appiah, Nagel, and Russell books cover fewer topics - basically, they hit some things that Appiah, Nagel, and Russell find interesting - and Plato can be tough to read on your own, which is why these are not my top recommendations, but they can serve as good introductions to the field of philosophy if you're starting from zero. The Appiah book is the most comprehensive of the three.

Next, a book that is often recommended to newcomers is Jostein Gaarder's book Sophie's World. It's a novel that also tries to introduce people to the various ideas of philosophy. I'm not the biggest fan of the book - its method of teaching philosophy sometimes reduces to "the main character receives a letter that contains a lesson on philosophy," which I find clunky - but many people enjoy it, so I would say it comes highly recommended except from me.

Finally, there's Andrew Bailey's First Philosophy series which sounds good but I've never read it.

##More Extensive Histories

If you're particularly industrious, or if you want to focus on non-Western philosophy, you could read more extensive books that are proper introductions to the history of philosophy. Will Durant's The Story of Philosophy and Anthony Kenny's A New History of Western Philosophy are good choices for Western philosophy, and Joel Kupperman's Classic Asian Philosophy and Brian Van Norden's Introduction to Classic Chinese Philosophy are good for Chinese and Asian philosophy. Victoria Harrison's Eastern Philosophy: The Basics covers India and China, and it's particularly short compared to the others - it sits somewhere between a proper history and the more summary-focused works noted above. Frederick Copleston has a ten volume History of Philosophy that is also good, although it's slightly hard to get ahold of these days.

With all of these histories of philosophy, there are three things to keep in mind. First, they are partial, both in the narrow sense (not even Copleston hits every important Western philosopher) and in the broad sense (philosophy from outside the West exists in all sorts of forms that don't show up in these books) so, if you'd like, you may want to start your investigation in an entirely different area. Second, philosophers disagree with each other over what philosophers in the past were saying, and although these histories are generally good about pointing this out, you should never take everything you read as the gospel truth. Third, lots of people find diving into a big long history (the Kenny book is 1000 pages!) to be pretty dry and boring. So, unless you're the sort of person who likes to read a lot of stuff, a history tome might not be a good place to start.

#More Specific Places to Start

We'll subdivide this into two sections. First, there are some good places to start for philosophy generally, if you're the sort of person who says "it's all interesting - I don't want to specialize!" Second, there are good places to start for various topics.

##Good Places to Start for Philosophy Generally

As mentioned above, Plato is a good place to dive in. He sits near the beginning of the Western philosophical tradition, his writing is often engaging, he covers practically every topic, and he was massively influential. In addition to this collection of five dialogues, Plato's Republic is a masterpiece that touches on many areas of philosophy and which has often served as the first introduction students get to the field.

The Meditations by Marcus Aurelius are focused on Stoicism, an overarching philosophy somewhat popular in Greek and Roman times. The somewhat personal style and the wide range of topics he touches on, plus the historical interest of reading something written not just by a philosopher but also an Emperor of Rome, make it a fun introduction to philosophy for many people.

Descartes was a revolutionary thinker who helped usher in the modern period. Two great places to start with Descartes are his Discourse on the Method and his Meditations on First Philosophy, both of which are available here.

##Good Places to Start for Specific Topics

We're subdividing again! One option is to go for primary sources - classic works in philosophy on these topics. Another option is to go for introductory textbooks or compilations of works.

###Primary Sources

Everyone has a favorite philosopher for any given topic, but it's always a different philosopher depending on who you ask. So, this section could balloon almost infinitely. But it won't. So, keep in mind that for every philosopher that's listed here, there are three others that someone might say "no, read THIS PERSON first!" That's life.

Moreover, philosophy is a super broad field. We cannot here cover every topic. That's also life.

This list skews slightly early in history, because it is generally better to start closer to the beginning. Sometimes people find it more exciting to jump in closer to the modern age. If that's the case, I recommend introductory textbooks listed below.

Finally, I am not super familiar with non-Western Philosophy. Unless someone with more knowledge comes along and offers some suggestions (thus allowing me to remove this disclaimer) the list below is very parochial. (Also, I've left off all of medieval philosophy since I think it's often boring for newcomers. Sorry. If you have some exciting medievals to suggest, I can add them to the list.)

For epistemology, which is the study of knowledge and how we come to have it, Plato's Theaetetus, Descartes's Discourse on the Method and Meditations on First Philosophy, Locke's Essay Concerning Human Understanding, Leibniz's New Essays on Human Understanding (read that right after Locke), Berkeley's The Principles of Human Knowledge, and Hume's An Enquiry Concerning Human Understanding.

For metaphysics, which is the study of what exists, what existence is, and why things do or don't exist, all that stuff up there in the epistemology section, minus Locke, Leibniz, and Hume, and plus Plato's Phaedo, maybe Aristotle's Metaphysics (it's tough), Leibniz's Discourse on Metaphysics and Monadology and Kant's Prolegomena to any Future Metaphysics.

For aesthetics, which is the study of beauty and art, Plato's Hippias Major, Aristotle's Poetics, Hume's The Standard of Taste, and if you're up for it, Kant's Critique of Judgment.

For ethics, Plato's Republic, Mozi's Mozi, Aristotle's Nicomachean Ethics, Hume's An Enquiry Concerning the Principles of Morals, and Kant's Groundwork for the Metaphysics of Morals.

For political philosophy, Plato's Republic and Crito, Aristotle's Politics, Cicero's On the Commonwealth and On the Laws, Machiavelli's The Prince, Hobbes's Leviathan, and Locke's Second Treatise of Government.

For existentialism and absurdism, Sartre's Existentialism is a Humanism and Camus's The Myth of Sisyphus and Other Essays.

For philosophy of science, which is the study of what science is and how science comes to know things, Mill's A System of Logic book III, Popper's Conjectures and Refutations, Kuhn's The Structure of Scientific Revolutions, and For and Against Method edited by Motterlini. Lots of people find Mill boring, though, and philosophy of science is rather new, so you might be better served by the textbooks or compilations listed below.

For feminism, Wollstonecraft's A Vindication of the Rights of Women, Mill's (or Mill and Taylor Mill's) The Subjection of Women, and de Beauvoir's The Second Sex.

For philosophy of religion, Plato's Euthyphro, Leibniz's Making the Case for God, Hume's Dialogues Concerning Natural Religion and The Immortality of the Soul, and Kant's Religion Within the Boundaries of Bare Reason.

###Introductory Textbooks

Maybe you want something that covers more ground faster, or that's pitched towards the modern reader, or which doesn't assume any prior knowledge. In that case, textbooks are good.

For epistemology, Robert Audi's Epistemology: A Contemporary Introduction to the Theory of Knowledge.

For metaphysics, Loux's Metaphysics: A Contemporary Introduction, Mumford's Metaphysics: A Very Short Introduction, Ney's Metaphysics: An Introduction, Carroll's An Introduction to Metaphysics, or Lowe's A Survey of Metaphysics. I know that's a lot of books - sorry. People have recommended one or the other at various times and I've read none of 'em. They all seem good though. Also Ortega y Gasset's Some Lessons in Metaphysics.

For existentialism, Bakewell's At the Existentialist Cafe.

For aesthetics, I don't really know, but these three seem good: Sheppard, Graham, and Stecker.

For ethics, Rachels' The Elements of Moral Philosophy, Sandel's Justice: What's The Right Thing to Do?, and Williams's Morality: An Introduction to Ethics.

For metaethics, which is basically philosophy about ethics that doesn't count as ethics, Chrisman's What is this Thing Called Metaethics?.

For political philosophy, Kymlicka's Contemporary Political Philosophy: An Introduction. Another option is Hampton's Poltiical Philosophy. See also Shapiro's The Moral Foundations of Politics and the Open Yale Course accompanying it.

For philosophy of science, Godfrey-Smith's Theory and Reality, Chalmers's What is This Thing Called Science?, and Barker and Kitcher's Philosophy of Science: A New Introduction. Lots of people also like Rosenberg's Philosophy of Science: A Contemporary Introduction but I don't.

For feminism, hooks's Feminism is for Everybody and Johnson's The Gender Knot.

For Buddhism, Gowans's Philosophy of the Buddha: An Introduction.

For philosophy of mind, Searle's Mind: A Brief Introduction.

For philosophy of religion, Davies's An Introduction to the Philosophy of Religion and Yandell's Philosophy of Religion: A Contemporary Introduction.

###Compilations of Works

One way to make quick buck is to grab a bunch of important stuff and stick it in a book, then sell it to people. Thankfully when philosophers do this, it makes it easy to get into a topic.

Epistemology: Epistemology: Contemporary Readings.

Metaphysics: Metaphysics: Contemporary Readings.

Aesthetics: Philosophies of Art and Beauty: Selected Readings in Aesthetics from Plato to Heidegger and Art and Its Significance.

Ethics: Ethics: Contemporary Readings and Ethical Theory: An Anthology.

Metaethics: Moral Discourse and Practice: Some Philosophical Approaches.

Applied Ethics (aka ethics about specific issues rather than ethical theories): What's Wrong? Applied Ethicists and their Critics.

Political Philosophy: Political Philosophy: The Essential Texts.

Philosophy of Science: Philosophy of Science: Contemporary Readings.

Chinese Philosophy: Readings in Classical Chinese Philosophy.

Philosophy of Mind: Philosophy of Mind: Classical and Contemporary Readings.

Philosophy of Technology: Readings in the Philosophy of Technology and Philosophy of Technology: The Technical Condition: An Anthology.

#Other Options

Rather than reading a book, you might want to start by watching some YouTube videos, listening to some podcasts, browsing around a philosophical encyclopedia, or, best of all, taking a philosophy course, either while you're in university or via something like a local community college or an online course (Shelly Kagan's course on death is very popular). These are great ways to get more acquainted with philosophy.

#Further Sources

https://www.reddit.com/r/askphilosophy/comments/48s7x6/metaphysicsepistemology_in_eastern_philosophy/

https://www.reddit.com/r/askphilosophy/comments/2jjqwn/looking_for_a_good_book_that_is_similar_to/

Much of the philosophy that is done today in America, especially by professional philosophers, sometimes gets labeled as either "analytic" philosophy or "Continental" philosophy. What do these terms mean? What is the difference between the two?

There is no single definition of these two terms that all or even most philosophers accept, and some people have more or less partiality for their own particular understanding of the split, so you should take everything, including what you read here, with a grain of salt. This answer, however, aims at being as capacious as possible, so to speak: it tries to capture the broadest and most obvious differences that pretty much everyone can agree on.

First we'll go through what these two terms mean, and then we'll touch on three main differences: the philosophers examined by each tradition, the style of writing, and sociological differences between the two.

#What are Analytic and Continental Philosophy?

Analytic philosophy is a tradition in philosophy that dates mostly to the sort of philosophy done in response to British Idealism, which was a movement of British philosophers like Green and Bradley in the late nineteenth and early twentieth century. People like Moore and Russell were not big fans of British Idealism, and the sorts of ideas and themes animating their rejection flowered into various forms, including logical positivism and ordinary language philosophy. This is a lot of jargon, but the basic idea is that some people started thinking about some things and framed a lot of questions in ways that still stick with us today. The name "analytic" comes from a desire on the part of people like Russell to engage in philosophy in what they took to be a more "scientific" way, that avoided large sweeping systems or what they took to be incomprehensible jargon in favor of precise focus on specific problems and straightforward use of words.

Continental philosophy is a tradition in philosophy that developed out of German Idealism, which was a response to Kant developed by people like Fichte, Schelling, and Hegel, and especially out of the response to the response, the main genesis of which was Husserl. Husserl's phenomenology led to many fruitful developments directly in phenomenology and in related fields like existentialism, driven by figures like Heidegger, Sartre, and Merleau-Ponty. This all developed into what is perhaps an even broader series of traditions than that sparked by analytic philosophy, traditions that still animate how we think today. The name "Continental" comes from the fact that the key figures of this tradition were on the European continent when it was developed (this despite many key figures of the analytic tradition also being European...).

One of the many places you could identify a split between analytic and Continental philosophy is right after Hegel. British Idealism was one reaction to Hegel, and Husserl and the tradition he spawned was another. The rejection of British Idealism spawned analytic philosophy, and analysis and critique of Husserl spawned Continental philosophy. Or, you could put the split after the rejection of British Idealism, because you might not think British Idealism counts as "analytic" enough for analytic philosophy. And so on. Dating the split is not an exact science.

#How are Analytic and Continental Philosophy Different?

As noted above, we'll go through three main differences.

##The Two Traditions Look at Different Philosophers

Speaking very broadly, up until Kant, analytic and Continental philosophers are all interested in everyone, but after Kant, there is divergence. (This is false - it is rare to find analytic philosophers talking about Machiavelli, Burke, Herder, or de Tocqueville, for instance. But it's a fine first approximation.)

After Kant, and especially after Hegel, Continental philosophers are very interested in the people who were influential on and the people influenced by the names mentioned above: Fichte, Schelling, Husserl, etc. Analytic philosophers, meanwhile, tend to ignore much of what happened between Kant and the genesis of analytic philosophy, except for Bentham, Mill, and Sidgwick. (Again, this is false. But it's a fine first approximation.)

This means that if you see a book or article written about Merleau-Ponty, it is likely written by a Continental philosopher, and if you see a book or article written about Quine, it is likely written by an analytic philosopher.

##The Two Traditions Sometimes Have Different Writing Styles

There is, I think, a bit more of an impetus for Continental philosophers to publish books, whereas analytic philosophers (although happy to publish books) are often quite article-focused. This is not a very large divide - both kinds of philosophers publish lots of books and lots of articles.

Analytic philosophy in its earliest form, and in many of its current forms, also developed a fairly distinct writing style that sounds like someone talking like they imagine scientists talk (this is not meant to be pejorative - I am an analytic philosopher and I love writing in this style). Much of this was a response to what analytic philosophers took to be obscurantist bullshit amongst Hegel and his followers (including the British Idealists).

Continental philosophy never developed a tradition of thinking that writing like Hegel turns everything into bullshit, and thus there is less resistance among Continental philosophers when it comes to writing prose that has a preponderance of terms and phrases that, on their surface, sometimes strike people as ornate or over-wrought.

##The Two Traditions are Made Up of Two Groups of People

People, especially people who work on philosophers later than Hegel, often end up divided into one or the other camp. If you go to a department that is mostly analytic philosophers (which is true about many American universities) you will be trained in analytic philosophy and potentially end up an analytic philosopher, and vice versa for Continental philosophy.

Because analytic philosophers read, talk to, and work with other analytic philosophers more than Continental philosophers, and because Continental philosophers read, talk to, and work with other Continental philosophers more than analytic philosophers, there are some sociological distinctions between the two camps.

Continental philosophy tends to have stronger ties with departments like Political Science (specifically, political theory - a topic for another day), Literature, Sociology, Rhetoric, and others. Analytic philosophy tends to have stronger ties with departments like Physics, Economics, Linguistics, Psychology, and others.

Classes on analytic philosophy often focus on reading articles and then dissecting the arguments by turning them into logical formulas and then attacking premises. Classes on Continental philosophy often focus on reading larger chunks of text, like for instance whole books, and tend not to be as hyper-focused on really nailing the author to the wall for screwing up in premise 6 or whatever.

#Honestly Though It's All a Jumble

As noted above, literally everything in this post needs to be taken with a grain of salt, not just because there is no general agreement on these topics but especially because at best these are just vague generalities that are false in almost as many instances as they are true. If we're counting by page numbers or years of existence, the vast majority of philosophy has nothing at all to do with analytic or Continental philosophy: most of it exists at a time prior to the split or in places like China and India that don't have anything to do with the split. Moreover, there are as many places that analytic and Continental philosophy overlap or come together as there are places they are apart.

#Further Reading

This article and this response, both written by philosophers, attempt to characterize the difference. If you compare them to this post, you will find some similarities and some differences, about which you are free to make up your own mind.

Some /r/askphilosophy threads on the topic, with some good answers:

https://www.reddit.com/r/askphilosophy/comments/19x7hv/what_is_the_difference_between_continental_and/

https://www.reddit.com/r/askphilosophy/comments/34v36n/what_is_the_difference_between_continental_and/

https://www.reddit.com/r/askphilosophy/comments/1qbh4h/difference_between_continental_and_analytic/

https://www.reddit.com/r/askphilosophy/comments/1yzepr/overview_of_continental_philosophy_vs_analytic/

https://www.reddit.com/r/askphilosophy/comments/1xrgt5/can_someone_eli5_the_difference_between_analytic/

One question people commonly wonder about is whether answers to moral questions can be "really" or "objectively" correct or incorrect. When I say something like "it's wrong to torture infants to death for pleasure" or "it's impermissible to enslave human beings for profit," am I right or wrong? If I'm right, am I "objectively" right, whatever this might mean?

These sorts of questions gain much more urgency in the face of moral disagreement. There are some topics in morality, like abortion, affirmative action, gay marriage, and immigration that people disagree vehemently about, both within societies and across societies. Moreover, if we look at societies in the past, we note even more disagreement: people once believed that slavery was morally acceptable. If there is so much disagreement about ethics, how can it be objective?

To answer this question we will look at three topics. First, what does it mean for morality to be objective or subjective? Second, does moral disagreement suggest that morality is subjective? Third, what other reasons are there for thinking morality is objective or subjective?

#What Is Objective Morality? What is Subjective Morality?

In philosophy, when we say that a statement is "objectively true" or "objectively false," or that it is "objective," we mean that it is true or false in virtue of facts that don't depend on what anyone thinks, feels, believes, desires, or anything like this. In other words, something is an objective truth if it's true no matter what's going on inside our heads.

Some examples of things that seem like objective truths are "the world is round," "spiders have eight legs," and "the speed of light is approximately 3.00×10⁸ m/s." These seem like statements that are true (or false!) regardless of what any humans happen to think. Even if I brainwash people into thinking that the world is flat, that spiders have fourteen legs, or that the speed of light is four meters per second, all I will accomplish is brainwashing people into having false beliefs about objective facts.

Meanwhile, statements are "subjectively true," "subjectively false," or just "subjective" if their truth or falsehood depends on what people think, feel, etc.

Some things that seem like subjective truths are "it costs $40 to stay in this motel for one night," "ethics class starts at 2:00 PM," and "the rules of chess say that the King can only move one square in any direction." These seem like subjective truths because they depend on beliefs that we have. If I brainwash everyone into thinking the motel costs $50 per night, that's what it will cost: there isn't some further, objective price out there. If I brainwash everyone into thinking class starts at 3:00 PM, that's when it will start: there isn't some further, objective time it starts out there. If I brainwash everyone into thinking the rules of chess allow the King to move two squares, that's what the rules of chess will be: there isn't some further, objective ruleset out there.

If you think about this too much, it actually starts to get pretty confusing and hard to tell subjective vs. objective statements apart. For example, if we looked up a chess rulebook printed before the brainwashing, it will say Kings only get to move one square. Who's right - the rulebook, or all of us? If we think the rulebook is right, then maybe the rules of chess are objective. (If the rules of chess are objective, it will probably turn out that morality is objective, too. Let's put this aside.) Hopefully, though, the distinction is clear enough for us to move on.

#Does Moral Disagreement Show that Morality is Subjective?

Notice first that we disagree about a lot of things that we don't think are subjective. Do vaccines cause autism? Did humans evolve from ape-like creatures? Was the Earth created 6,000 years ago by god? Will raising the minimum wage hurt the economy? Is global warming caused largely by human actions? These all seem like questions with objective answers: whatever the right answer is, it doesn't depend on anything we happen to believe. But there is lots of disagreement about the right answer. So this suggests that disagreement doesn't tell us anything about objectivity or subjectivity, at least on its own.

This is not to say that disagreement is no challenge to objectivity. We might think that we have good procedures for clearing up disagreement on certain topics, but we don't have procedures for clearing up disagreement in ethics. Or we might think that disagreement on certain topics goes away over time, whereas disagreement in ethics sticks around more or less forever. Or we might think that there is just much more disagreement about ethics than about other topics.

It's not clear that any or all of these are good arguments. There are also reasons to think that what appears to be ethical disagreement is not in fact ethical disagreement. Consider the debate over abortion. It may turn out that what people are really arguing about is a non-ethical issue, namely, whether the fetus has a soul or is otherwise a "full" person. The ethical question is whether we can kill the fetus, but if we agree that the fetus is a full person, maybe everyone will agree it's wrong to kill it, and if we agree that the fetus isn't a full person, maybe everyone will agree it's okay to kill it. Religious and scientific disagreement causes us to differ on whether the fetus is a full person, which causes us to have moral disagreement. But we don't disagree about the moral principle: everyone agrees that it's wrong to kill full persons.

In general, what's called the "argument from disagreement" is not a super popular argument for the subjectivity of ethics among philosophers. This is not to say it's obviously false, though. We have covered just a tiny stretch of the argument from disagreement. For a defense of the argument, John Mackie's book Ethics: Inventing Right and Wrong is the most famous source. For a very good response to the argument, see this article by David Brink.

#So Is Morality Objective or Subjective?

That was just a small taste of the sorts of arguments philosophers have about moral objectivity. That Brink paper discusses one other common argument for moral subjectivity: the "argument from queerness," which is the argument that objective morality is just too weird of an idea to be true. We could go on listing arguments for and against objective morality for quite a while.

To jump to the chase, there are lots of philosophers who support the idea of objective morality, also known as moral realism. They do so in the form of theories like moral naturalism and moral non-naturalism. There are also plenty of philosophers who argue that morality is subjective. This view is also known as moral anti-realism.

Moreover, there are positions that fall in between the two sides, or that are difficult to categorize as one or the other. Does moral constructivism argue that ethics is objective or subjective? It's kind of an open question!

Suffice it to say that there are very good arguments on pretty much every side of the debate, encompassing arguments for and against basically any objection you can come up with. As this other FAQ answer points out, moral realism is hardly a fringe position. So, although we can't say anything definitive, we can say that nobody is obviously or even likely ruled out.

#What Can I Do With a Philosophy Degree?

Like most bachelor's degrees granted by liberal arts institutions, a philosophy bachelor's degree is not direct preparation for any sort of job. (Even a bachelor's degree like Computer Science or some kind of Engineering degree is often insufficient preparation for a job as a coder or an engineer.) This means two things.

First, a philosophy degree does not uniquely qualify for you for any sort of job - you will never really see a job listing specifying "bachelor's degree in philosophy or equivalent job experience" or anything like this.

Second, a philosophy degree is fine preparation for any sort of job that requires a general bachelor's degree, which includes a wide variety of jobs in fields like consulting, marketing, and even technical fields like computer science, which are often more interested in your skills and the presence of any degree than they are in what specific degree you have. A person with a philosophy degree and a good portfolio of open-source programs and good performance on the job interview coding test will beat out a computer science major with no portfolio and middling performance on the test any day of the week.

This means that if you are focused on getting a job, a philosophy degree can be as good a choice as any other bachelor's degree, especially other bachelor's degrees that don't lend themselves to particular jobs (like humanities or social science degrees).

In general, if your goal is to be employable, the typical advice applies, which has nothing to do with philosophy: network and meet people so that you have access to job openings; become accomplished at selling yourself and describing your skills in ways that are attractive to employers; acquire work experience in the form of internships or jobs so that you have skills, references, and a history of being employable; and so on.

One area in which a philosophy degree can be helpful is in helping you develop certain skills that are attractive to employers (especially if you are good at pitching the skills as attractive). Philosophy degrees often teach people to read and write very well: you have to understand complicated texts, explain complicated ideas, and effectively convey arguments in writing. Many employers are looking for employees who are able to do this. You don't need a philosophy degree to learn these skills, and these skills can be learned in other university courses, and if you don't have good professors, you might not even learn these skills by studying philosophy - but, this can be the sort of thing you get from a philosophy major.

#Should I Major in Philosophy?

This depends as much on the kind of person you are, the kind of professors at your institution, the requirements for majoring in philosophy at your institution, the sort of job you want, and other idiosyncratic features as it does on anything we can speak to here.

However, many people find philosophy to be a very rewarding topic to study. If you find you enjoy the field, it can be quite worthwhile to major in it. This website has some resources discussing the value of studying philosophy.

To be perfectly honest, majoring in philosophy isn't going to get you much more than a piece of paper (although to be fair, majoring in anything for a bachelor's degree doesn't get you much more than a piece of paper). Your university's philosophy department will certainly appreciate it if you major in the topic rather than just taking some classes, though, and since you have to major in something, there's nothing wrong with majoring in philosophy!

#What do Philosophy Majors Do?

Philosophy majors do lots of things. Some end up attempting to do philosophy professionally, which entails graduate school and then teaching philosophy as a professor. See this FAQ answer for more information on philosophy graduate school.

Most philosophy majors don't try to become philosophers, though. They go on to jobs in many different fields. Philosophy majors tend to score very well on graduate school entrance exams like the GRE (for most graduate schools), the GMAT (for business school), and the LSAT (for law school). Among non-STEM majors, philosophy majors tend to earn more money than others (although this may be due to the sorts of people who major in philosophy, rather than any effect attributable to a philosophy degree). So, a philosophy degree is perhaps good preparation for whatever you want to do with the rest of your life. It may even help you figure out what you want to do with the rest of your life!

#Further Reading

https://www.reddit.com/r/askphilosophy/comments/3oqt7n/should_i_become_a_philosophy_major/

https://www.reddit.com/r/askphilosophy/comments/o5wj8/for_those_of_you_who_have_majored_in_philosophy/

https://www.reddit.com/r/askphilosophy/comments/16ri9i/can_i_find_a_job_with_a_ba_in_philosophy/

https://www.reddit.com/r/askphilosophy/comments/1oq08u/why_should_i_major_in_philosiphy/

https://www.reddit.com/r/askphilosophy/comments/353ful/people_with_a_bachelors_degree_in_philosophy/

https://www.reddit.com/r/askphilosophy/comments/36uhxs/quick_question_what_can_you_actually_do_in_the/

https://www.reddit.com/r/askphilosophy/comments/3j6z7a/with_a_degree_ba_in_philsophy_what_could_i_do/

https://www.reddit.com/r/askphilosophy/comments/3j7r65/is_studying_philosophy_a_good_idea/

https://www.reddit.com/r/askphilosophy/comments/3kom63/what_job_could_i_expect_to_do_one_day_if_i_become/

https://www.reddit.com/r/askphilosophy/comments/3mr1p0/those_of_you_with_a_degree_in_philosophy_what_is/

https://www.reddit.com/r/askphilosophy/comments/420km0/what_to_do_with_a_major_degree_in_philosophy/

https://www.reddit.com/r/askphilosophy/comments/483l8i/another_question_about_job_prospects/

##Meta Note (Added After Posting) As is made evident by the upvote/downvote count on this post and on various replies below, and by various other replies below, Sam Harris is rather popular on reddit among non-philosophers. That is in fact why this FAQ question is here - when redditors find out that philosophers don't share their love of Harris, questions often arise. This FAQ question is not a place to substantiate accusations against Harris in any detail - the goal here is just to mention them in enough detail to show why philosophers have problems with him. If, like many redditors, you don't have problems with him, you're welcome to downvote me or argue in the comments below, but this FAQ post is not going to engage with you in any detail. Again, just to be clear as crystal, the purpose of the post is to briefly describe what philosophers find objectionable about Harris to clear up confusion. It may be that you disagree with philosophers. That's fine! Harris himself disagrees with philosophers. This is not really the place to argue about all that. Also, for the sake of transparency, I should not that I've edited "drone bomb" to "nuclear bomb" below in the "Harris is Racist" section and I added a link behind the words "self-proclaimed neuroscientist" to explain the genesis of that phrase.

#Sam Harris

Sam Harris is a self-proclaimed neuroscientist and popular author on various topics, including philosophical topics. He is also a prominent atheist. Philosophers tend not to be big fans of Sam Harris. There are four main issues that philosophers have with Sam Harris. The first is that Sam Harris is racist. The second is that Sam Harris makes bad philosophical arguments. The third is that Sam Harris makes disingenuous philosophical arguments. The fourth is that Sam Harris denigrates philosophy in a manner philosophers find objectionable. Let's go through all four of these.

#Harris is Racist

Harris is racist - specifically, he's an Islamophobe who thinks that we ought to do terrible things to people with brown skin from predominantly Muslim countries, like nuclear bomb them, torture them, and racially profile them. Whether it's objectionable to hold these views is a substantive moral debate which we won't go into here - suffice to say that reasonable people often come down opposed to Harris on these topics, and if you disagree, then we've identified a way in which you think philosophers unnecessarily dislike Harris.

This topic is also somewhat controversial because Harris often denies that he is committed to these positions, going so far as to edit blog posts he's made (without giving any indication that he has edited them) to back away from these sorts of positions (while at the same time continuing to espouse them elsewhere). If you don't think Harris engages in this sort of subterfuge or you find it unobjectionable, then, again, instead of hashing this whole thing out, suffice it to say that you differ from philosophers on this point.

In general, this is not the forum to make any sort of case against Harris on these topics. This would require surveying the available evidence (a task complicated by Harris's subterfuge) and providing substantive moral arguments against Islamophobia. These would both require more space and effort than is available here. You are welcome to conduct your own investigation and form your own opinions. This is just a place to note the reasons philosophers have for finding Harris objectionable, and his Islamophobia is one main reason.

#Harris Makes Bad Philosophical Arguments

Harris's work on free will is not particularly philosophically sophisticated. Daniel Dennett, one of the other most prominent popular atheists (and also a respected philosopher of much more philosophical acumen than Harris) has a good article on this topic.

One of the main mistakes that Harris makes is a mistake that many undergraduates typically make when first exposed to the topic of free will, which is to reject compatibilism (the most popular position on free will among philosophers) for failing to be about what free will "actually" is - the sort of free will that ordinary people think of when they think of free will. There are lots of reasons to think Harris is simply wrong about this - some are discussed here and here (PDF). Moreover, as Dennett points out, this is hardly dispositive when it comes to the free will debate. It may be that ordinary people aren't very sophisticated about free will, and further investigation into the topic will show that compatibilism is a much better way to understand free will.

Harris's mistake here is not just large in the sense of being fairly indefensible (although it is) - it's also large in the sense that it is not a very sophisticated mistake. His main argument against compatibilism is not one that we find in the philosophical literature, it's one we find amongst undergraduates who have yet to grasp the debate. Even philosophers who agree with Harris's conclusions about free will do not advance Harris's arguments about free will, because they are terrible arguments.

#Harris Makes Disingenuous Philosophical Arguments

In addition to free will, Harris has written on morality. Here, his work is not even substantive enough to count as bad. Instead, Harris's work on morality consists largely of deceptive redefinitions of terms and unsupported assertions of positions that have been investigated by philosophers in detail for decades.

Harris deceptively redefines terms by turning all inquiry into science. This post on Harris's blog is the best admission of this redescription. There he claims that "We must abandon the idea that science is distinct from the rest of human rationality." In effect, any time you are "adhering to the highest standards of logic and evidence, you are thinking scientifically." This of course means that one need not be engaged in anything like what anyone typically considers "science" to be doing science. Philosophers, for instance, turn out to be engaging in science when they do philosophy (so long as they do it much better than Harris). Police detectives trying to solve a murder are scientists, as are people trying to figure out which dog pooped on the floor, farmers deciding which crops to grow to make money, economists doing economics, sociologists doing sociology, literary critics engaged in literary criticism, and basically anyone who isn't being illogical or ignoring reality.

If we redefine science like this, it turns out science can tell us quite a bit about morality, says Harris. Often this gets shortened to something "science can solve morality," which is the substantive position Harris claims to defend. But once we've expanded science to include (for instance) philosophy, it's trivial to point out that "science" can tell us about morality. This just amounts to saying that philosophers can tell us about morality. Certainly it doesn't imply that one ought to ask an actual scientist, that is, someone in a science department at a university, about morality. They are no more likely to be an expert about morality than the farmer or the person investigating dog poop.

The second main issue with Harris's approach to morality is that (ignoring his redefinition of science) he tries to reduce morality to a scientific problem in another sense: he says that morality is all about maximizing well-being, and science can tell us what maximizes well-being.

This is, all-told, not a crazy view. Many respectable philosophers hold approximately this view. It is a form of consequentialism and it has a long, storied history which you won't learn about if you read Harris, who ignores this long storied history.

The issue with Harris is that his argument in favor of the view consists simply of asserting that it is true. Here is Harris's argument from The Moral Landscape:

The concept of “well-being” captures all that we can intelligibly value... “morality” — whatever people’s associations with this term happen to be — really relates to the intentions and behaviors that affect the well-being of conscious creatures.

This is, as noted, not a strange or outlandish position. It does, however, face strong objections. One of the most famous objections goes something like this: imagine that there has been a murder in a small town. Coincidentally, a stranger has just arrived in town. The sheriff knows that the murder cannot be solved: the culprit won't be caught because there is not enough evidence, although he does know that the stranger is innocent. People in the town are suspicious of strangers, especially the recently arrived stranger, because he's of a different race than the townsfolk (he's black, they're white). They're convinced he's the murderer and they're marching, in a mob, to lynch him for the murder.

The sheriff has two options. He can use the police force to protect the stranger, at the cost of the townspeople violently rioting, which will result in many deaths, although the stranger will be safe. Or, he can frame the stranger for the murder, appeasing the townsfolk, which keeps them from lynching him or rioting. The stranger will be prosecuted and sentenced to life in prison, or death, or something similar. Should he frame the stranger?

Many people think the answer is "no," or at least it's not obviously "yes." It seems unjust to frame the stranger. However, it will maximize well-being to frame the stranger - the stranger's conviction will result in a loss of well-being, but not as much as would be lost in the violent, bloody riot.

This is exactly the sort of case that philosophers argue about in order to defend or attack something like Harris's position. Harris doesn't bother responding to this sort of case, or in fact any plausible counterargument to his view. (He does address various counterarguments, but they are awful counterarguments that no philosopher has ever advanced - they consist of straw man positions like "what if someone thinks that dying early and painfully is better than living a long happy life?")

Thus the main issue with Harris's moral views is not that they are implausible - it's that he does not argue for them, he simply asserts them, even though he acts as if he is engaging in meaningful philosophical inquiry and substantively defending his position. In philosophy we are interested not in what someone can assert with no argument but rather in what someone can plausibly argue for. Because Harris cannot plausibly argue for his view that well-being is all that matters, morally speaking, Harris has not presented a compelling view of ethics.

#Harris Denigrates Philosophy

Let's look at a quote from the above-mentioned book:

Many of my critics fault me for not engaging more directly with the academic literature on moral philosophy. There are two reasons why I haven’t done this: First, while I have read a fair amount of this literature, I did not arrive at my position on the relationship between human values and the rest of human knowledge by reading the work of moral philosophers; I came to it by considering the logical implications of our making continued progress in the sciences of mind. Second, I am convinced that every appearance of terms like “metaethics,” “deontology,” “noncognitivism,” “antirealism,” “emotivism,” etc., directly increases the amount of boredom in the universe.

Philosophers might find this sort of talk objectionable for two reasons. First, Harris suggests that he is not at all indebted to moral philosophy for any of his views. Given the generally uninformed and poorly-defended nature of his views, we might take him at face value when he says he hasn't learned anything of substance from reading philosophy, but a philosopher might still feel slighted that, having taken a look at the field, Harris has rejected it in favor of what he calls "the logical implications of our making continued progress in the sciences of mind." Ignoring for the moment the fact that, as noted above, he has already redefined "sciences of mind" to include philosophy, we might think that the view that "sciences of mind" are the way to answer these questions rather than philosophy objectionably excludes philosophy from a realm of inquiry to which it is uniquely suited. Philosophers, understandably, may find this offensive.

Second, Harris here denigrates terms that pop up in moral philosophy fairly often, because he finds them boring. Philosophers might feel that this does not properly respect the reason these sorts of terms exist - just like science (in the sense of actual science, not in Harris's understanding of science) uses many complicated words, like "deoxyribonucleic acid," not for the sake of being boring but rather for the sake of being precise and accurate, philosophy uses terms like "metaethics" not for the sake of being boring but for the sake of being precise and accurate. Harris's assertion that these terms do nothing but put people to sleep (beyond revealing much about the degree to which he gleans any sort of understanding from writing which employs these terms) suggests that he thinks philosophers are really just being boring for the sake of being boring. Whether he's right or not, it's probably understandable that some philosophers would find this objectionable.

#Further Reading

##Racism

https://www.schneier.com/essays/archives/2012/05/to_profile_or_not_to.html

http://www.salon.com/2016/03/07/my_secret_debate_with_sam_harris_a_revealing_4_hour_dialogue_on_islam_racism_free_speech_hypocrisy/

http://www.salon.com/2014/09/06/richard_dawkins_sam_harris_and_atheists_ugly_islamophobia_partner/

http://www.theguardian.com/commentisfree/2013/apr/03/sam-harris-muslim-animus

##Free Will

https://www.reddit.com/r/askphilosophy/comments/23nxi8/ive_read_harris_free_will_and_i_cant_find_flaws/?

https://www.reddit.com/r/askphilosophy/comments/1379by/any_good_critiques_of_sam_harris_and_free_will/?

https://www.reddit.com/r/askphilosophy/comments/1x5yyq/discussion_about_dennett_and_harris_on_free_will/?

https://www.reddit.com/r/askphilosophy/comments/42waw0/whats_wrong_with_the_arguments_and_opinions_in/?

##Morality and Disingenuous Definitions

https://www.reddit.com/r/askphilosophy/comments/4bxw83/why_is_badphilosophy_and_other_subs_in_reddit_so/

https://www.reddit.com/r/askphilosophy/comments/26p4iv/what_are_some_knockdown_objections_to_sam_harris/?

https://www.reddit.com/r/askphilosophy/comments/25teiz/is_sam_harris_considered_a_bad_or_controversial/

https://www.reddit.com/r/askphilosophy/comments/28f9pe/is_the_morality_or_ethics_proposed_by_sam_harris/?

https://www.reddit.com/r/askphilosophy/comments/oemcs/raskphilosophy_what_is_your_opinion_on_sam/?

https://www.reddit.com/r/askphilosophy/comments/1s8pim/rebuttals_to_sam_harris_moral_landscape/?

https://www.reddit.com/r/askphilosophy/comments/36le8j/why_is_there_so_much_hatred_for_sam_harris/?

https://www.reddit.com/r/askphilosophy/comments/20gmqr/sam_harris_moral_theory/

https://www.reddit.com/r/askphilosophy/comments/1bcd6f/why_isnt_sam_harris_a_philosopher/

##Etc.

http://www.reddit.com/r/askphilosophy/comments/6h17jp/do_you_think_sam_harris_is_doing_a_good/

https://www.samharris.org/blog/item/the-limits-of-discourse

Perhaps you've read the "Is Free Will an Illusion? FAQ post here, and have decided to endorse an incompatibilist hard deterministic view. Does that mean that your choices don't matter at all, and that you have no control over your life? Should you just sit down and let physics take its course? Does hard determinism imply fatalism?

TL;DR: No, determinism and fatalism are distinct positions. Even if you believe that all your beliefs, desires, and actions are the result of deterministic physical processes, there's still some sense in which you can be said to have control over your actions. Determinism does not imply that you ought to become completely passive and "let fate take its course," because even in a deterministic universe, your actions are part of what shapes the future.

Recall the distinction between determinism, hard determinism, and compatibilism. All hard determinists are also regular determinists. The difference between determinism and hard determinism is that hard determinists believe that free will in any sense is incompatible with determinism, and that determinism is true. That is, hard determinists are determinists who also reject compatibilism, and so who think we lack free will. Many people who believe in some form of free will (even if just in the weak sense of our actions being a consequence of our desires, and not coerced) are also determinists (including me); they (we) just think that the conditions that are necessary for having some kind of free will are compatible with a world in which the future is fully determined by the past. Hard determinists reject that idea: they're determinists, and they think determinism excludes the possibility of free will.

However, there's an even stronger position here: fatalism. Fatalists believe that in the face of some determined outcome (either because determinism is true in general or because some particular outcome is inevitable), it's reasonable to conclude that your choices don't matter, and that you don't have any control over the world--that outcomes don't depend on actions. This sounds a lot like hard determinism, so I think the distinction is best illustrated with a story.

Suppose you and I are caught in a major earthquake. We are, of course, both quite concerned that we might die in the quake. We're also both hard determinists, so we think that there's no "genuine" free will involved in any of our actions; whether or not we're both going to die in this quake is, in a sense, outside our control.

When the quake starts, we're walking down the streets of Los Angeles. A new skyscraper is under construction nearby, and they're currently hoisting a new sheet of window glass up for installation in one of the top floors. As the shaking begins, you say

Quick, we need to get out from under that huge sheet of glass and under cover if we want to survive!

I look at you like you're crazy, and just stay where I am, watching the glass swing precariously on its rope. I tell you:

What's the point? If we're going to die in this quake, we're going to die in this quake. The universe is totally deterministic, and my choices don't make any difference.

You shrug your shoulders and run off, getting under a doorframe and waiting things out. I stand there eating an ice cream cone as the rope breaks, the pane of glass comes down, and cuts me neatly in half. You survive the quake, and run over to steal my wallet and ice cream cone.

In this story, we're both determinists, but I'm also a fatalist. I refuse to recognize that there's a causal link between actions that I take (or fail to take) and the outcome of certain events, and that this is true whether or not that causal link includes anything that might be properly termed "free will." It's true that your survival and my death are both consequences of physical laws (or whatever), but they're also consequences of differences in the actions that we took.

The fatalist mistakes a lack of freedom for a lack of influence or control over future outcomes. It may be true that the past determines the future, but that doesn't imply that my actions don't "make a difference" in the sense of playing a role in the causal chain that leads from the past to the future. After all, if the past uniquely determines the future and my actions are part of the past, then my actions play a role in determining the future. When I said "my choices don't matter" in that little story, I (as a fatalist) meant that literally. Fatalists believe that your actions have no causal influence on the future.

The important distinction to make here is between believing that the past uniquely determines the future (i.e. that there's a bijective correspondence between past states and future states) and beliving that a particular future state is inevitable (i.e. that all past states lead to a particular future state). In cases where the latter is true, then fatalism makes sense; there's no reason to work to avoid a future state that can't be avoided. However, in virtually all real-world cases, we have no good reason to think that every course of action leads to the same future. Because we don't generally know the exact correspondence between our actions and future states--that is, because we can't predict the future with perfect precision--it makes sense to behave in a way that we think is most likely to produce a future state that we desire. It makes sense, for instance, to attempt to find shelter during an earthquake even if whether you live through the quake or die in it is a consequence of deterministic laws. The fatalist either rejects (or fails to understand) the idea that because your actions are part of the causal chain of events that shapes the future, it's possible to have a meaningful sense of control over that future, even in a deterministic universe. Fatalism is, unsurprisingly, not a popular position (possibly because most people who hold it die in earthquakes or the like).

This doesn't necessarily mean that fatalists reject moral responsibility. It's possible to believe that your actions are determined (or even that they don't matter in any robust way) and still blame people for what they do. It's not a common position, but it is one that some people do hold (theological Calvinists believe something like this).

Further reading:

• The FAQ post on free will and determinism

• The Determinism and Freedom Philosophy Website

• The SEP on fatalism

• This thread in /r/askphilosophy

• Determinism vs. fatalism

What do philosophers think about moral realism?

Surprisingly, a slim majority of philosophers are “moral realists”: they think that there are some objective moral facts. The 2009 PhilPapers survey asked just under a thousand philosophers and philosophy graduate students about moral realism, and discovered that 56.4% were moral realists, 27.7% weren’t, and 15.9% held some other position. Isn’t 56.4% a pretty small majority? Well, among philosophers it’s actually quite significant. Only about eighty percent of philosophers were prepared to say that they believed in the existence of the external world, for instance: ten percent denied it, and ten percent held some other position. In any case, for every philosopher who thinks there aren’t any objective moral facts, two philosophers think there are. This result isn’t indicative of philosophers being religious, either. The same survey found that just under fifteen percent of philosophers accepted or leaned towards theism. Over seventy percent were atheists, and twelve percent held some other position. So quite a lot of philosophers think that there are moral facts but don’t think that God exists.

Does this represent a worrying consensus for the person who thinks there aren’t any objective moral facts? Yes, it does, and it’s worse than it initially appears. The skeptic thinks that there obviously aren’t any objective moral facts. But even philosophers who are committed to moral anti-realism think that there are some good reasons to be a moral realist. They don’t think that proponents of objective morality are just confused, rhetorically sneaky, or crypto-theists. Unfortunately, there is no study on whether philosophers think that moral realism is obviously false - in part because many philosophers would find the question too silly to answer. But if the question was not “is moral realism true” but “is there a good case to be made for moral realism”, I suspect the percentage would jump from 56.4% to somewhere in the high nineties. The moral skeptic will certainly be able to find philosophers who agree with him that there aren’t any objective moral facts. However, he won’t be able to find many philosophers who agree with him that moral realists are all horribly confused. He might not be able to find any.

Arguments for moral realism

I’m going to quickly run through short versions of two standard arguments for moral realism, and some standard responses to common arguments that skeptics put against moral realism. Let’s start with some arguments for moral realism.

Argument from taste: Even if we call ourselves moral anti-realists, our attitude to moral preferences is significantly different from our attitude to ordinary preferences. If I don’t like noodles, it doesn’t make much sense for me to say “I’m glad I wasn’t born in China, because then I would probably like noodles”. But it makes perfect sense to say “I’m glad I wasn’t born in the Middle Ages, because then I would think the sun revolved around the earth.” And it makes perfect sense to say “I’m glad I wasn’t born in antebellum America, because then I would probably support slavery”. So it looks like we treat our attitude towards slavery more like a matter of empirical fact than a matter of mere preference. This argument is lifted wholesale from David Enoch, who calls it the “spinach test”. Given that, our intuitive starting point seems to be some kind of moral realism. Of course, our intuitive starting point might be wrong! But if it is, we’ll need to be persuaded to abandon it. We shouldn’t assume that moral anti-realism is the default view and expect moral realists to convince us otherwise.

Argument from plausibility: When we’re deciding what to believe, we should try to only start with the premises we’re most confident in. If a premise seems a bit dubious, we should take a step back to a safer one. But our confidence in at least one moral proposition seems to be greater than our confidence in any of the arguments for moral anti-realism. Take the claim “it is objectively wrong to torture your infant son to death for fun”. To me, this claim seems to be as secure as what I can see with my own eyes. In fact, it seems more so: if I somehow became convinced that either I was hallucinating or torturing my infant son to death for fun was right, I would immediately assume I was hallucinating. This claim certainly seems more secure than claims like “moral realism is a bit weird”, or “if people disagree about morality, there might be no right answer”. This is a gloss on arguments made by G.E. Moore and Michael Huemer. Of course, a knock-down proof of moral anti-realism should give me pause. But if there’s no knock-down proof available, I’ve got no reason to abandon a premise I’m very secure in for a premise that just seems plausible.

Note that neither of these arguments depend on God. So far we’ve established that moral realism is an attractive position, and that we need some actual reasons against it if we’re to reject it.

But what about...?

Let’s address some common reasons against moral realism now. As we’ll see, none of these reasons are strong enough to rule out moral-realism.

The evolution objection: We can explain our moral intuitions by evolution. Given that, isn’t it silly to think that they’re connected to the truth? Note that we can explain our intuitions about physics by evolution too, and we all agree that they’re loosely connected to the truth: objects fall down, throwing something hard makes it move quickly, and so on. The fact that our moral intuitions evolved doesn't automatically mean that moral realism must be false, or that our moral intuitions can’t be connected to the truth.

The disagreement objection: People disagree a lot about morality, and different cultures have very different ideas about what’s morally acceptable. Given that, isn’t it silly to think that there’s one moral truth? First, disagreement about morality is a bit overblown. Pretty much everyone agrees that there’s something morally wrong with torturing children for fun, that we ought to keep promises, that being kind is usually better than being cruel, and so on. Second, areas of apparent moral disagreement, such as the arguments over gay marriage, often rest on a disagreement about non-moral matters: for instance, whether same-sex parenting causes children psychological distress. Third, disagreement about a topic isn’t itself a reason to think that there’s no truth there. People disagree about physics, especially between cultures, but nobody takes that to be a reason to doubt physics. Most people - or everyone - could just be wrong.

The strangeness objection: It makes sense to say that we should eat if we don’t want to be hungry, or that we should be kind if we want to be liked. But it’s very weird to say that we should be kind to people full stop. This looks like a different, strange sense of the word “should”. Isn’t it a bit too strange to be plausible? Note that there’s one other area in which this unconditional sense of “should” gets used: talking about truth and evidence. It’s natural to say that we should only believe what we’ve got evidence for, or that we should try to believe true things even if we’d be happier believing false ones. If these statements aren’t too strange, then saying “we should be kind to people” isn’t too strange either.

TL;DR

People who think there aren’t any objective moral facts ought to admit that they’re holding a position that a (slim) majority of experts disagree with. They shouldn’t treat moral realism as if it were obviously wrong, or as if it were already settled to be false. Most philosophers are moral realists, and there are good responses to the standard arguments many people give against objective moral facts.

For more information, see this introductory Enoch paper, this paper about evolutionary arguments against moral realism, or this summary article about moral realism.

Moral theories tell us what to do. Sometimes they tell us to do things we don't want to do. For instance, a moral theory might tell us that animal cruelty is wrong, and we shouldn't support animal cruelty, but I really want to eat a hamburger. This leads to a natural question: why should I be moral?

There are, broadly, two different responses philosophers have given to this question. The first response is to claim that the question makes no sense. The second is to claim that the question makes plenty of sense. Let's go through these responses.

#The Question Makes No Sense

According to some philosophers, the question "why should I be moral" makes no sense. We can rephrase it to "why should I do the thing that is the right thing to do?" or "why should I do what I ought to do?" and these two questions are obviously nonsense. The fact that something is the right thing to do is already a reason to do it. You don't need another reason to do the right thing.

Similarly, we might say that if you accept the truth of mathematics, asking "why should I get 4 when I add 2 and 2?" is nonsensical. 2+2 just is 4 - there's no other reason you should get 4 when you add 2 and 2 beyond the fact that 2+2=4.

So, according to these philosophers, you should be moral because being moral is what you should do.

#This Question Makes Perfect Sense

Other philosophers think that the question "why should I be moral?" makes plenty of sense. They think that for you to have a reason to do something, there has to be something about you that makes it your reason. So for instance we might think that "it's the right thing to do" is only a reason for you if one of the following things is true:

• You want to do the right thing.
• You want to avoid opprobrium, and opprobrium will accompany doing the wrong thing.
• You would feel bad if you acted immorally.
• You don't want to go to Hell, and you believe this will happen if you act immorally.
• Doing the right thing will impress someone you want to impress.
• Etc.

Notice we could list a zillion reasons, but also notice that none of these reasons will necessarily be there. That is, it's at least possible that there could be a person for whom "it's the right thing to do" doesn't give them a reason to do it.

One example is the psychopath. There is disagreement in philosophy and psychology about what psychopathy consists of, but one way of understanding psychopathy is to understand it as a condition according to which the psychopath doesn't care about moral reason just because they happen to be moral reasons. So, the psychopath never cares about doing the right thing, unless doing the right thing is good in other ways.

But, as we pointed out above, doing the right thing isn't always good for you. Sometimes it's very hard to do the right thing. The psychopath, in a situation like this, might say "I don't care about morality! I'll do what's best for me. I will eat a hamburger." According to the first view we examined, the psychopath is making a mistake, but according to the second view that we're looking at right now, the psychopath isn't making a mistake. She's morally bad, of course, but she has no reason to be morally good.

Who is Right?

Unfortunately this debate isn't really going anywhere, if you ask me. We've more or less reached a stalemate, I think, between the two sides.

Here's one argument people in the second camp (call them "externalists") give against people in the first camp (call them "internalists"). Externalists say: "the psychopath is a perfectly sensible person. She's a terrible person, of course, but she's not making any sort of rational mistake. She's not confused about anything. She doesn't misunderstand morality. She just happens not to care about it. And since she doesn't care, she has no reason to be moral in this case."

Internalists respond: "the psychopath doesn't actually understand morality. She talks as if she understands it, but if she truly understood what the words 'right' and 'wrong' and 'moral' and 'immoral' meant, she'd realize she's making a mistake here, and ignoring the reasons she has to do the right thing, just like if someone understands what '2' and '4' and '+' and '=' means, they'd realize they're making a mistake when they say they don't see why they have to get 4 when they add 2 and 2."

For a recent attempt to help reconcile the two perspectives and make internalism more attractive, you can check out Peter Railton's paper "Internalism for Externalists."

#Does it Matter?

In reality, the internalists and the externalists don't disagree about lots of real-life cases, we might think. Most of us are decent human beings who don't want to be evil. Moreover, there are lots of other reasons to be moral, like the criticism that other people will provide if you do the wrong thing, or the fact that if you're a bad person, people will often be bad to you in return. So for practical purposes, the distinction between internalism and externalism is not always a huge deal. Even psychopaths have reasons to keep from being immoral all the time, because this will often turn out bad for them.

Moreover, we might think that morality itself doesn't really care about the distinction. In other words, moral theories tell us what actions are right and wrong, just like math theories tell us what math answers are right and wrong, and if we have no reason to be moral unless it serves our interests, this doesn't tell us anything interesting about morality, just like if we have no reason to be mathematically correct unless it serves our interests, this doesn't tell us anything interesting about math.

This is, however, a very interesting philosophical question that gets right to the heart of what it is to have reasons to do things. For the internalist, we do learn something about morality: it's a standard which gives reasons to everyone, no matter who they are or what they care about. For the externalist, meanwhile, we can't always guarantee that we'll have reasons to be moral, which means that it might sometimes be reasonable to be immoral, according to the externalist. These are both interesting results.

#Further Reading

This Stanford Encyclopedia of Philosophy Page addresses the topic in detail.

Here are some /r/askphilosophy threads on the topic:

https://www.reddit.com/r/askphilosophy/comments/2rdmw4/why_should_i_be_moral/

https://www.reddit.com/r/askphilosophy/comments/29ok3s/why_shouldnt_i_do_whatever_i_want/

https://www.reddit.com/r/askphilosophy/comments/2omg7z/why_should_i_do_good/

https://www.reddit.com/r/askphilosophy/comments/1w314t/why_act_ethically/

https://www.reddit.com/r/askphilosophy/comments/1xn4aj/am_i_obligated_to_be_ethical/

https://www.reddit.com/r/askphilosophy/comments/285rnp/why_does_why_should_i_be_moral_make_sense_as_a/

https://www.reddit.com/r/askphilosophy/comments/3ihafk/why_should_an_individual_care_about_the_well/

Consequentialism is the moral theory that says we should judge the rightness and wrongness actions based on their consequences. One question people often have is whether every moral theory can be turned into a consequentialist moral theory: just take whatever the moral theory says is wrong, and say "those are bad consequences," and take whatever the moral theory says is right, and say "those are good consequences," and you're done, right?

This is actually a somewhat complicated question, for two reasons. First, it's not clear that consequentialism vs. whatever is a good way to divide up moral theories. Second, granting that the division makes sense, there is debate as to whether every moral theory can be consequentialized. Let's go through both topics.

#Is Consequentialism a Relevant Category?

When we first learn about moral theories in introduction to ethics courses, for instance, consequentialism is often presented as one alternative, with deontological ethics and virtue ethics as the two most popular alternatives.

These are good ways to introduce students to moral theories, but there are reasons to think that, once you dig very deeply and start thinking about the structure of morality, these divisions aren't picking up on anything fundamental or important. Something that looks more relevant is the divide between theories that admit only of agent-neutral reasons vs. theories that also admit of agent-relative reasons. Agent-neutral reasons are reasons that don't make any reference to the person they apply to: "maximize utility" is an agent-neutral reason because it doesn't have anything to do with the person it applies to. Agent-relative reasons make reference to the person they apply to. "Take care of your children" is an agent-relative reason because it tells you to take care of your children, not just children generally. It gives different reasons to different people, because different people have different children.

Consequentialism seems like a kind of moral theory that only works with agent-neutral reasons. "They're my kids!" is not a reason for a consequentialist to take care of her kids. Deontological theories, meanwhile, seem like they are all about agent-relative reasons. "Don't murder anyone" is the sort of rule that deontological theories might tell us to follow no matter what, even if your refusal to murder causes someone else to murder five people. A consequentialist, when faced with this choice, would say that "don't murder anyone" is an agent-neutral reason, and the best way to make sure it gets fulfilled is by murdering someone so that someone else doesn't murder five people. The deontologist, though, would take "don't murder anyone" to be agent-relative: you aren't supposed to murder anyone, and it's not your job to worry about what other people do.

If the divide between agent-neutral and agent-relative reasons is one of the fundamental divides in moral theory, it makes more sense to talk about moral theories in terms of how they approach that topic, rather than in terms of consequentialism or anything else.

#Consequentialization

Let's put all that aside, though. A second issue is that philosophers disagree about whether any moral theory can be consequentialized. One of the most famous papers on this topic is Campbell Brown's "Consequentialize This." In addition to agent-neutrality, Brown identifies two other features of morality that he think are essential to consequentialism: no moral dilemmas and dominance.

No moral dilemmas means consequentialism is committed to the idea that you are never in a situation where every choice is immoral.

Dominance means that if you have two choices, one of which is morally correct according to consequentialism, the correct one is always going to be morally correct compared to the incorrect one whenever you're faced with any situation where these two choices exist.

Moral theories without these three features, Brown things, can't be consequentialized.

This is just one proposal about consequentialization. Some philosophers disagree with Brown and think that everything can be consequentialized, or they disagree with Brown about what makes something consequentializable. Suffice it to say that it's not obvious that everything can be consequentialized, and it's also not clear that this is something we necessarily have to worry about.

#Further Reading

Here are some /r/askphilosophy threads on the topic:

https://www.reddit.com/r/askphilosophy/comments/1y9ovq/true_or_false_every_nonconsequentialist_moral/

https://www.reddit.com/r/askphilosophy/comments/2o65cj/ive_heard_people_say_that_just_about_any_ethical/

https://www.reddit.com/r/askphilosophy/comments/3gpbp7/how_does_consequentialization_work_how_would_you/

https://www.reddit.com/r/askphilosophy/comments/43vnmn/wondering_how_ethical_theories_dont_all_just_fall/

https://www.reddit.com/r/askphilosophy/comments/30odop/kant_as_a_consequentialist/

'Philosophy' is a word that has a lot of meanings in English.

Etymologically, the word comes from Greek, where it was formed from 'philo' (love) plus 'sophos' (wisdom) and where it meant something like "a love of wisdom." That usage is no longer current.

Today, 'philosophy' sometimes means "a way of living life" or "a rule that you follow." So for instance someone might say "my philosophy is to live and let live" or "my philosophy is never to go to bed hungry."

The most relevant usage of the term for our purposes, though, is to refer to the study of a certain set of issues. 'Philosophy' in this sense consists of the sorts of things philosophers study and talk about. It's what you learn in Philosophy departments in universities. Figuring out what 'philosophy' means in this sense will help us answer our questions. Let's call this kind of philosophy 'academic philosophy.'

#Academic Philosophy

Unfortunately it turns out to be very hard to come up with a good definition of what academic philosophy consists of. Probably the best we can do is list and describe the various topics that philosophy investigates.

There are lots of things academic philosophy investigates. It ranges from classic topics like metaphysics (the study of what exists, of necessity and possibility, and other topics), logic (the study of logical reasoning), epistemology (the study of knowledge and the ways we can acquire it), ethics (the study of what is right and wrong, good and bad, vicious and virtuous, etc.), aesthetics (the study of beauty and art) - to the more recent topics, like the philosophy of computer science and the philosophy of economics.

What unites all these topics? What makes them academic philosophy rather than something else? One problem is that academic philosophy overlaps with other disciplines in various areas. Theoretical physicists and philosophers often discuss the same topics, publish in the same journals, and cite the work of each other, which can make it hard to tell philosophy apart from physics. Political philosophy overlaps with work done in political science departments under the name political theory. Fields like game theory, which used to be obviously just academic philosophy, are sometimes considered to be their own field, too.

Nevertheless, some philosophers have tried to establish criteria for what makes something academic philosophy, like "philosophy is whatever can be discovered without doing any experiments or other investigation into the world" - that is, you could sit in an armchair and discover all of academic philosophy if you thought hard enough - or "philosophy is the field that lays the foundations of science" or "philosophy is the most basic kind of inquiry." There is no general agreement among philosophers on any of these answers, though.

So for now, let's just say that academic philosophy is the study of various sorts of topics that philosophers have long taken themselves to be concerned with.

#What do Philosophers do?

Professional philosophers spend their days reading, writing, talking about, and teaching academic philosophy. Philosophers publish their work in journals, like Philosophy & Public Affairs and The Philosophical Quarterly, and in books. They read what others publish and comment on it, in their own writing and in person. Philosophers often give and attend talks where ideas are presented, like works in progress or recently published papers, and they go to conferences on topics where lots of people present works in progress and talk things over with each other. Philosophers are almost always employed as professors by universities in Philosophy departments.

#Who is a Philosopher?

Of course, you don't need to be a professional philosopher to do academic philosophy. Do you have to be a professional philosopher to be a philosopher? Just like there is no accepted definition for what academic philosophy is, there is no accepted definition for who counts as a philosopher. Are stand-up comics who talk about philosophical issues philosophers? They're certainly not professional philosophers - they don't do academic philosophy for a living - but are they amateur philosophers?

Again, there is no clear answer to this question. The best we can do is probably look to how words are typically used. What kinds of people are generally referred to as philosophers?

#Further Reading

The /r/philosophy FAQ begins with a description of what "philosophy" is and isn't.

Here are various /r/askphilosophy threads on this topic:

https://www.reddit.com/r/askphilosophy/comments/3fqeav/what_is_philosophy/

https://www.reddit.com/r/askphilosophy/comments/33jsji/i_dont_understand_philosophywhat_is_philosophy/

https://www.reddit.com/r/askphilosophy/comments/2j1d4l/what_is_philosophy_in_lame_mans_terms/

https://www.reddit.com/r/askphilosophy/comments/1vdvm8/put_simply_what_is_philosophy/

https://www.reddit.com/r/askphilosophy/comments/1zvhj4/what_exactly_are_the_aims_and_values_of_philosophy/

https://www.reddit.com/r/askphilosophy/comments/2x8w32/what_is_philosophy/

https://www.reddit.com/r/askphilosophy/comments/4i069y/question_for_those_that_hold_the_title_of/

It turns out to be very hard to come up with good arguments that justify eating meat. This is for two reasons. First, the arguments in favor of vegetarianism/veganism are much stronger than the arguments in favor of eating meat. Second, it's not clear that we should think about this in terms of arguments in favor of eating meat, because that might be the wrong way to look at it. Let's go through these two things.

#Arguments for Vegetarianism/Veganism are Good, Arguments in Favor of Eating Meat are Bad

There are some very strong arguments for vegetarianism/veganism. One of the best articles on this topic is Alastair Norcross's "Puppies, Pigs, and People" (PDF). Norcross argues that just like we'd react in horror to discovering someone who tortures puppies to death in order to be able to eat chocolate, we should react in horror to our own practices, which amount to torturing pigs, chickens, cows, and other animals (including dogs, in countries outside the West) to death in order to be able to eat meat. Since animal cruelty seems clearly wrong, arguments like Norcross's are very compelling.

Meanwhile, arguments in favor of eating meat look quite suspect. Four common arguments are "it's necessary for health," "it's natural," "it tastes good," and "animals do it." Let's go through all four.

##It's Necessary for Health It is simply false that eating meat is necessary to maintain one's health in the vast majority of cases. In affluent Western countries, nobody is in danger of starving to death. Vegetarian and vegan diets can provide all the necessary nutrients and can support the exact same lifestyles as meat eating can. There are reasons to think that vegetarianism or veganism can even be healthier in many cases.

##It's Natural

The idea that "natural" actions are okay faces two main issues. The first is that it's unclear exactly what "natural" means. Is it "unnatural" to refrain from eating meat? Why? The second and much larger issue is that it's not clear why something being natural makes it morally acceptable. Natural and moral are two entirely different properties. Saying "it's natural, so it must be okay!" is like saying "it's related to peanut butter, so it must be okay!" or "it's done on a Tuesday, so it must be okay!" The fact that an action relates to peanut butter or occurs on a Tuesday doesn't tell us whether it's morally acceptable or not. Why should the action's status as natural tell us whether it's morally acceptable or not? Lots of natural behavior, like rape and murder, are paradigmatically morally unacceptable behaviors.

##It Tastes Good

The fact that meat tastes good is not a great argument for thinking that it's moral to eat meat, for the same reason that how good it feels to rape or murder someone tells us nothing about whether it's okay to rape or murder someone. Notice also that if "meat tastes good" is a good argument for eating non-human animals, it ought to work for eating humans too, but it seems objectionable to say that there's nothing wrong with grilling up some baby back ribs made from actual human babies.

##Animals Would Eat Us if They Could, and They Eat Each Other

Animals do all sorts of things to each other (and to us), including eating each other. Does this make it okay for us to do the same thing? There are three main issues with this.

The first is that not all animals eat each other - cows, for instance, are herbivores, so it seems a little unfair for us to eat them on the basis of other animals eating each other.

The second, much bigger issue is that animal behavior doesn't excuse our own behavior, because animals aren't able to understand or act according to morality. Like infants, they aren't responsible for what they do. The fact that an infant poops and vomits all over me doesn't show that it's okay for me to poop and vomit all over you. Unlike the infant, I have the ability to make moral choices.

Finally, even if animals were moral agents just like us, this wouldn't make it okay for us to do bad things like the animals. If my neighbor kills and eats innocent people, this doesn't make it okay for me to kill and eat innocent people.

#This is the Wrong Way of Looking at it

The second issue with arguments in favor of eating meat is that it's not clear that we should approach the debate like this. Imagine that someone asks "what are the best arguments for thinking it's morally acceptable to use my left hand to pick up a glass of water and drink it?" Your response would probably be "that's not really a moral question, but uh I guess it's okay unless it would result in something bad happening or something?"

Meat eating is like this. Unless there's something wrong with meat eating, it's not clear that it's a moral question at all. We can say the same thing about other foods. Unless there's something wrong with eating bananas, it's not clear that it's a moral question at all.

Of course, there are many reasons to think eating meat is a moral question, as noted above. However, the way to understand the debate is like this: if those arguments are right, then eating meat is wrong. If the arguments are incorrect, though, then we don't need extra moral reasons for thinking eating meat is okay. If those arguments are wrong, we're just at the default position, which is to say that eating meat is like eating a banana: there's nothing much to talk about.

#Further Reading

https://philpapers.org/rec/FISAFC

Some /r/askphilosophy threads on this topic:

http://www.reddit.com/r/askphilosophy/comments/29rpfy/are_there_any_convincing_arguments_for_meateating/

http://www.reddit.com/r/askphilosophy/comments/1hoc3z/whats_a_good_argument_in_favour_of_meateating/

http://www.reddit.com/r/askphilosophy/comments/22y31d/is_there_any_moral_justification_for_being_a/

https://www.reddit.com/r/askphilosophy/comments/33ne0a/question_regarding_ethics_and_the_consumption_of/

https://www.reddit.com/r/askphilosophy/comments/3ud38k/if_meat_isnt_needed_for_health_why_is_it_morally/

https://www.reddit.com/r/askphilosophy/comments/42of0d/philosophy_seems_to_be_overwhelmingly/

https://www.reddit.com/r/askphilosophy/comments/3e4955/if_it_is_not_ok_is_be_cruel_to_an_animal_why_is/

Here are podcasts that have been recommended by /r/askphilosophy posters in the past, ordered vaguely from my favorites to my least favorites. I have not listened to most of them, and these are just my own opinions, so you should not take the ordering very seriously at all. If you want a nearly complete list of philosophy podcasts, see here.

• The History of Philosophy Without Any Gaps
• The Philosopher's Arms
• Political Philosophy - Open Yale Courses
• The UnMute Podcast (iTunes)
• Hi-Phi Nation
• In Our Time: Philosophy (iTunes)
• A Pod Called Quest
• Free Will Matters
• The Free Will Show
• Jurisprudence Course
• Overthink
• New Books in Philosophy
• What's Left in Philosophy
• Hotel Bar Sessions
• Neurodiving
• Consciousness Live!
• sutras (and stuff)
• Philosophy and Film Podcast
• Philosophy Bites
• Always Already Podcast
• How To Think About Science
• Minerva Podcast
• Modern Day Philosophers
• Philosophy 6, 001 Fall 2010 UC Berkeley by Hubert Dreyfus (Professor Dreyfus has other philosophy courses on iTunesU too)
• The Partially Examined Life
• The Philosopher's Zone
• Oxford Philosophy
• Very Bad Wizards
• Elucidations
• Philosophize This!

One thought that commonly occurs to people is that everything we do is somehow selfish. Every time we think about an action that we take, it looks like we can discover some selfish motive for it. For instance, why did I eat that peanut butter and jelly sandwich? Because I wanted to be less hungry. Why did I study for that test? Because I wanted to get a good grade so I can get a degree and get a good job. Why did I give money to that charity? Because I wanted to feel good about myself. This view is known as psychological egoism, and the vast majority of philosophers think it is false. Here are some reasons to think that psychological egoism isn't right:

#Counterexamples

The most obvious reason to think that psychological egoism is false is that many people seem to do things that aren't actually selfish. So for instance a soldier might jump on top of a grenade to save their fellow soldiers, even though this results in their own death. Someone might devote their entire life to helping the sick and the poor. Someone might bravely report on the injustices of the government even though they know that they are likely to be tortured and killed. Someone might vote for a tax increase that will cost them money they could've spent on a new yacht, even though the taxes will go towards helping other people get medical care.

These seem like obvious cases where someone is engaged in behavior that isn't selfish. So psychological egoism seems like it is clearly false.

A defender of psychological egoism might reply that really what is happening is that these people do have selfish motives. The soldier wants to be remembered as a hero, the person serving the sick and the poor wants the sick and the poor to like them, the journalist really likes writing articles about corruption, the person voting for the tax gets a warm fuzzy feeling from voting for that tax.

There are two main issues with this response. The first is that it seems false in many cases. It's true that a soldier could jump on a grenade in order to be remembered as a hero, but there's no good reason to think that this is why all soldiers jump on grenades. Similarly, even though I could eat a peanut butter and jelly sandwich because I think this will summon aliens from another dimension, there's no good reason to think that this is why most people generally eat peanut butter and jelly sandwiches. If we use the normal tools of psychology and philosophy to investigate the motives people have for their actions, it is simply false that these tools always tell us that their motives are selfish.

There is a second, deeper worry about this response, though, and it gets to another main issue with psychological egoism.

#Triviality

Let's say you could describe everything anyone does with a selfish motive. The problem with this is that you have to stretch the idea of a "selfish" motive so thin that it no longer tells us anything interesting. Before you started reading this post, if I had said "selfish," you probably would have pictured someone who cares more about themselves than others, who is liable to accept a small gain for themselves even if this imposes a large cost on others, and so on. This idea of selfishness is the one that describes an interesting character trait, one which some people have and other people don't.

If we save psychological egoism by showing that the soldier, the faithful servant of the sick and poor, the brave journalist, and the taxpayer are all technically selfish, what have we really done? It turns out that "selfish" now isn't a very descriptive term at all. Imagine I invent a new word, "smellfish," and I say that all human beings act from smellfish motives, 100% of the time. You ask me what this means, and I say that no matter what action you come up with, I can generate a smellfish motive. Smellfishness doesn't mean anything much more than that - it's just a property that attaches to every action.

I take it you would be very unimpressed with my discovery that all humans are smellfish. Learning that Jane is smellfish tells us nothing about what she's going to do. Smellfishness is a psychologically and philosophically worthless idea.

If that's what psychological egoism forces us to do with selfishness, psychological egoism is not a helpful viewpoint.

#Everyone Wants to Do What They Do

At this point the psychological egoist might reply that psychological egoism must be true - if someone takes an action, clearly something in them wanted to take that action. If they didn't want to do that action, they wouldn't have done it, of course! And doing what you want is a form of selfishness.

This just brings us back to our two above responses. The first is to note that it seems false that people only do what they want to do. I grade the papers my students turn in, not because I want to but because I have to. I take out the trash when it's full, not because I want to but because I have to. And so on.

The psychological egoist could reply that of course I "want" to do these things in some sense - I want not to get fired from my job as a teacher, I want my apartment not to smell like trash, and so on. Unfortunately this brings us right back to triviality. When I first used the word "want," you probably had in mind something like "enjoy" or "desire" or "look forward to" - if someone says "I really want to take out the trash!" in normal conversation, you'll look at them funny. We could redefine "want" to mean "anything you do is something you want to do," but then we aren't saying anything interesting anymore.

These reasons, and more, are why most philosophers think psychological egoism is a dead end.

#Further Reading

See this article, which was linked above, and these reddit threads:

https://www.reddit.com/r/askphilosophy/comments/2a6cda/is_a_person_who_enjoys_helping_others_selfless/

https://www.reddit.com/r/askphilosophy/comments/168aop/any_opinions_on_psychological_egoism/

https://www.reddit.com/r/askphilosophy/comments/25qt2j/can_people_act_outside_of_selfinterest_opinion/

https://www.reddit.com/r/askphilosophy/comments/1qcwwg/do_people_only_operate_on_selfishness_as_their/

One common worry people have is that humans have no free will, because everything we do is determined by the laws of physics. Way back when the universe began, a bunch of particles started moving around, and since then all those particles have obeyed the laws of physics, including the particles in your brain and your body. This suggests that we never freely choose to do anything, and that the impression of free choice we have is an illusion: whenever we make a decision or move a limb, really what is happening is that the particles in our brain and our body are obeying the laws of physics and doing the only thing they could have done.

In more specific terms, the idea is that the truth of causal determinism renders free will impossible. (This is different from whether God's knowledge of everything we do undermines free will and it is also different from whether everything we do is bound by fate. On the topic of fate, see this FAQ post.)

In philosophy, there are three main responses to this question.

One option is to accept that causal determinism is true and to accept that this makes free will impossible. This is sometimes known as "hard determinism" or "incompatibilism." I will call it "hard determinism" here.

A second option is to accept that causal determinism is true, but argue that this doesn't make free will impossible. This is known as "compatibilism."

A third option is to deny that causal determinism is true, and argue that because it's false, we have free will. This is often known as "incompatibilism" or "libertarianism" (not to be confused with political libertarianism). (The reason "incompatibilism" can refer to this view and also the first view is that an incompatibilist says that determinism is incompatible with free will. There is then a further question about whether determinism is true.)

#Hard Determinism

We have already seen what the argument for hard determinism looks like. The laws of physics determine everything we do, which means we aren't the ones who determine what we do, which means we have no free will. Hard determinism is not a very popular view among philosophers. One of the most interesting results of hard determinism is that it looks like it undermines moral responsibility. Without any free will, how can we be responsible for anything we do? The laws of physics made us do it. So if I steal your money and chop off your arm, it would be wrong to punish me, just like it would be wrong to punish me for something someone else did. It's not my fault!

#Compatibilism

The second option is compatibilism, which tries to save free will in the face of determinism. The compatibilist generally makes their argument by showing that whatever characteristics of free will that you think are important, they can either exist under determinism, or they aren't actually important. Let's look at two examples: making decisions that accord with your desires, and the ability to do otherwise.

##Acting in Accord with your Desires

One important part of free will is that the decisions we make aren't random, so to speak. If I see someone say "my favorite food is peanut butter and jelly sandwiches!" but then they decide to eat a potato instead of a peanut butter and jelly sandwich, I would start to worry about whether they freely chose that potato. Maybe someone forced them to eat the potato. In general, if our choices and actions flow from our desires, intentions, goals, and personality, they seem like they are our actions, but if they come from an outside source (someone forcing you to do something, mind control, etc.) they don't seem like they're freely chosen anymore.

The compatibilist points out that determinism doesn't undermine this sort of freedom of choice. Even if determinism is true, if I choose to eat a peanut butter and jelly sandwich, I can do it because I like peanut butter and jelly sandwiches. It's true that not all of our choices are like this. I didn't choose what my favorite food was, nor did I choose to be born, or anything like this. But that's okay. We still have free will when we make most choices, according to the compatibilist. So, we've saved one important aspect of free will from determinism.

##The Ability to Do Otherwise

Another feature of free will we might think is important is the ability to do otherwise. If I eat a peanut butter and jelly sandwich, but it was determined by the laws of physics that I would do this, then I never had any other option, right? So how could that choice be free?

The compatibilist can argue something like this: imagine that there is no determinism, and that Jane's deciding whether to rob a bank. She doesn't know that an evil scientist is watching her every move. Jane is driving a van, and when she comes to the crossroads, she will either decide to turn left and rob the bank, or turn right and give up her life of crime. If the evil scientist sees her start to turn right, the scientist will use a mind control ray and make her rob the bank anyways. Jane comes to the crossroads, and because she's kind of a jerk, she decides to turn left and rob the bank. The scientist never has to do anything.

Jane couldn't have done otherwise, so to speak. She had to rob the bank. If she had chosen not to, she would have been mind-controlled into doing it. The only possibility open to Jane was robbing the bank. But, did she freely choose to rob the bank? The answer seems like it's "yes" - the scientist never had to do anything. Jane made that choice on her own.

But notice now that we don't seem to think that you have to be able to do otherwise in order to choose freely. This means that even if determinism is true, and we can't do otherwise, we might still choose freely. One defense of this position is Harry Frankfurt's famous essay "Alternate Possibilities and Moral Responsibility."

So, those are two sorts of arguments for compatibilism. There are many more. Most philosophers are compatibilists.

#Incompatibilism

Finally, let's move on to the incompatibilists. (See also here.) For the incompatibilist, humans are able to break free of determinism somehow, either because the universe isn't deterministic in the first place or because we're special for some reason. Incompatibilism has a tough hill to climb, because many people think the universe is either deterministic or at least close enough that humans can't break free from the laws of physics. About as many philosophers endorse incompatibilism as endorse hard determinism.

#Further Reading

For further reading, see the articles linked above, this book, this website, and the following threads in /r/askphilosophy:

http://www.reddit.com/r/askphilosophy/comments/33187x/are_there_any_modern_proponents_of_free_will/

http://www.reddit.com/r/askphilosophy/comments/32wira/can_someone_explain_to_me_how_compatibilism_is/

http://www.reddit.com/r/askphilosophy/comments/324p0l/do_you_believe_in_free_will/

http://www.reddit.com/r/askphilosophy/comments/31ssvf/where_to_start_with_free_will/

http://www.reddit.com/r/askphilosophy/comments/2jwnbr/what_makes_free_will_free_to_the_compatibilist/

http://www.reddit.com/r/askphilosophy/comments/1r8c84/do_we_have_no_free_will_at_all_or_could_we/

http://www.reddit.com/r/askphilosophy/comments/338kjt/i_dont_see_how_free_will_can_exist/

https://www.reddit.com/r/askphilosophy/comments/3dktjd/i_dont_think_i_understand_compatibilistism/

https://www.reddit.com/r/askphilosophy/comments/3dh850/do_we_have_free_will/

https://www.reddit.com/r/askphilosophy/comments/3depzl/i_want_to_learn_more_about_free_will/

https://www.reddit.com/r/askphilosophy/comments/3d4df5/any_credible_arguments_for_free_will/

https://www.reddit.com/r/askphilosophy/comments/3blq1s/whats_the_problem_with_determinismcompatibilism/

https://www.reddit.com/r/askphilosophy/comments/3bi996/i_do_not_believe_in_free_will_can_anyone_provide/

https://www.reddit.com/r/askphilosophy/comments/39aydj/can_you_use_cause_and_effect_to_argue_against/

https://www.reddit.com/r/askphilosophy/comments/38qpkh/what_are_the_arguments_for_the_presence_of_free/

https://www.reddit.com/r/askphilosophy/comments/38nwr5/can_a_strict_materialist_or_naturalist_believe_in/

https://www.reddit.com/r/askphilosophy/comments/38dguo/arguments_have_been_made_about_free_will_for_ages/

https://www.reddit.com/r/askphilosophy/comments/3f15kj/how_candoes_free_will_exist/

https://www.reddit.com/r/askphilosophy/comments/3qqwk9/can_philosophy_answer_the_question_is_there_free/

https://www.reddit.com/r/askphilosophy/comments/3rmpw4/ive_always_wondered_is_there_no_free_will_in_the/

TL;DR Kaufmann's translation of the Genealogy of Morals is a fine place to start; don't worry too much about reading other authors first.

In what order should I read Nietzsche?

There are two schools of thought here. School 1 holds that you should read Nietzsche chronologically, i.e.:

1. The Birth of Tragedy
2. Untimely Meditations
3. Human, All Too Human
4. The Dawn aka Daybreak
5. The Gay Science
6. Thus Spoke Zarathustra
7. Beyond Good and Evil
8. On the Genealogy of Morals
9. The Case of Wagner
10. Twilight of the Idols
11. The Antichrist
12. Ecce Homo
13. Nietzsche contra Wagner

School 2 holds that you should instead begin with some of the easier works, such Beyond Good and Evil, Genealogy of Morals, or Twilight of the Idols, before moving on, if you're still interested, to the rest of the Nietzsche's work.

My personal opinion is that the the second school is preferable in most cases. If you're certain that you'll be reading all of Nietzsche's corpus, then the chronological path is probably superior. Otherwise, you'll be better served by starting with the works you'll get most out of: you don't need to read all of Nietzsche to understand his central tenets, and you'll get a good chunk of his central tenets from any of the three books mentioned.

You'll notice that this list does not include The Will to Power. Contemporary scholars have determined that the book of that name was heavily edited by Nietzsche's anti-Semitic sister. This does not mean that the aphorisms contained within are not worth reading, but it is better, if possible, to read them in the appropriate context and with some background in Nietzsche already in hand.

What translations should I buy?

Kaufmann and Hollingdale are both fine, and are likely to be the ones you will find most cheaply. If you're certain that you're going to do scholarly work on Nietzsche, then the recent Stanford editions are the best translations available, and the Cambridge editions are serviceable. Kaufmann's likely remain the easiest and most pleasurable to read.

What other authors should I read first?

Full disclosure, I'm of the opinion that there are so many things that one could read that you should start somewhere, and Nietzsche is a fine enough place to start. However, a familiarity with the basic tenets of the Presocratics,--particularly Heraclitus--Socrates, Plato, Kant, and Schopenhauer will be extremely helpful for particular parts of Nietzsche's corpus. Additionally, knowledge of the work of Wagner and Goethe is essential for understanding Nietzsche's discussion of these figures, and Thus Spoke Zarathustra is very purposefully modeled on the Bible.

What about secondary sources?

There's a substantial amount of literature on Nietzsche, and much of it of low quality. I've broken the list up into rough groups that attempt to indicate why one might want to read the source in question.

Influential interpreters whose views pre-date sixty years of research into Nietzsche's manuscripts: Deleuze, Heidegger, Hollingdale, Kaufmann, .

Influential second-wave English interpreters: Danto, Magnus, Maudemarie Clark, Nehemas, Schatt, Solomon.

Contemporary interpreters of note: Christa Davis Acampora, R. Lanier Anderson, Jessica Berry, Maudemarie Clark, Ken Gemes, Nadeem Hussain, Christopher Janaway, Paul Kastafanas, Brian Leiter, Simon May, Bernard Reginster, John Richardson, Simon Robertson, Julian Young. (I've probably missed a couple.)

If you're looking for an overall introduction, Julian Young's Friedrich Nietzsche is likely to be your best starting place. In a pinch, however, either Kaufmann's or Hollingdale's intellectual biography will do, though the reader should recognize that both are now dated.

This thread is intended to be a running list of questions that probably should be added to the FAQ, but which haven't yet been addressed. If you have a question you'd like to see answered (but don't feel qualified to answer it yourself), head over to this thread in /r/AskPhilosophy and post a request. If you're an approved contributor here, feel free to create a "stub" answer post here with the question, and tag it with the "Request" flair. Approved contributors can reply with their answers, and a mod will move them into the body of the post and remove the "request" tag. Alternatively, you can reply to this post with your request and I will add it to this list.

Suggested Questions:

• Something explaining the continental-analytic split

• Solipsism/External world realism/other minds

• Empiricism, scientism, and epistemology

• Logical positivism and its downfall

• Marx and Hegel misunderstandings (this one might be too broad/big to be even a pair of posts)

• Nominalism and platonism in mathematics

• On the nature and purpose of proofs of god/first cause, etc.

• Where should I start as a beginner to philosophy?

• Something about depression/feeling worthless/suicide? /r/philosophy has a little blurb in the sidebar, and maybe something similar directing people to a more appropriate sub would be enough.

• Something about nihilism

• Ayn Rand / objectivism

• Why moral realism?

• What's wrong with Sam Harris?

• What can I do with a philosophy degree?

• Is free will an illusion? (introducting compatibilism/incompatibilism)

• How can I make a choice if determinism is true? (distinguishing determinism from fatalism)

Should you pursue a PhD or MA in philosophy? What are the most important parts of your application? How should you choose a school?

TL;DR: Honestly, you probably shouldn't go. This isn't intended to crush your dreams, but the reality is that your chances of getting into grad school are pretty low and the job market is terrible. It's a risky proposition, and it might end up being a huge waste of time.

1. General Remarks

Before I say anything else, I should emphasize that my knowledge and experience is with mainstream North American philosophy departments (thanks for pointing this out, /u/PlausibleApprobation) . Some of what I say here is likely applicable more broadly, but probably not all of it. If you're a student outside North America (especially in non-Anglophone nations), applying outside North America, or work on extremely non-mainstream topics, your mileage may vary.

Undergraduate admissions are really just about identifying well-rounded students who are likely to do well in general, and so lend themselves naturally to more objective measurements like grades and test scores. In most cases, if you're a very strong applicant to an undergraduate program, you're going to get admitted. PhD admissions are really different, though. By the time you get to that level, it's expected that virtually all serious applicants would be able to do the work (at least in an abstract "are-you-smart-enough-for-this" sense). A good school might get several hundred applications for six or seven slots each year, and the vast majority of those applicants are "smart enough" to warrant admission, at least in theory. Cutting the field down to the final 15 or so who will be offered admission is therefore much less about the sort of objective metrics that are important for undergraduate admissions, and much more about subjective evaluations by people on the admission committee. They'll evaluate you in terms of how well your interests fit into the department, whether or not there's someone who is willing and able to spend the requisite time supervising your work, how much they need new people to TA certain classes, and so on.

Even with all that, it's a virtual guarantee that there will be more suitable applicants than there are available slots, so to some extent whether or not you're admitted is a matter of luck of the draw. Most of the time, the committee is seeking to create an incoming cohort with balanced and diverse interests, so if you happen to be the only strong applicant with a particular focus who applied that year, you'll have an easier time (assuming that the department and faculty are prepared to supervise someone with your interest). If, on the other hand, there are 25 other people with the same interest as you this time, well, you just have to toss the dice. This is why I like to really emphasize that getting rejected--even from a lot of places--is not necessarily a negative reflection of your abilities. In contrast to undergraduate admissions, PhD admissions are as much a function of the department's needs and desires as they are of your qualifications. That's why it's important to apply to lots of places; I ended up getting rejected from a number of schools with significantly weaker programs than the one I actually enrolled in, and that's true of most people I know.

Even if you do get accepted, remember that the academic job market for philosophers right now is really really really terrible. You should think long and hard about whether or not this is the path you really want to take, and whether or not you'll be willing to spend 6+ years getting a PhD, then another indeterminate number of years floating around looking for a good job. That's the best case scenario if you want to be an academic. It takes a lot of patience, and can be really demoralizing, so be prepared for that if you choose to proceed. Getting a PhD--even from a top-tier institution--is no guarantee that you'll land any job at all right after graduation, much less a good tenure-track job somewhere you like. Caveat philosopher.

Still here? OK. If you decide to go forward, then here's what you should know. The most important things (in order of significance) are (1) your writing sample, (2) your letters of recommendation, (3) your GRE scores, and (4) your GPA and the quality of your undergraduate program. (1) and (2) are far and away more significant than (3) or (4) when it comes to your chances of being accepted. Graduate programs (especially at the PhD level) are looking to identify whether or not you have potential as a researcher and scholar, which is indicated far more strongly by the quality of your writing and what your undergraduate professors have to say about you than it is by your test scores or undergraduate grades.

2. Writing Sample

Approach a professor that you know well and for whom you've written a very strong paper (a senior thesis project or the like is great, but a strong term paper is fine too). Tell them that you're interested in applying to grad school, and ask if they think it would be worth your time to try to turn the thesis (or part of it) into a writing sample. If they say that it would be, ask for some advice about what you might do to improve the quality of the paper, and if they'd be willing to take a look at (and comment on) a revised draft once you've had time to prepare it. Take their criticism seriously--treat it like comments from a peer reviewer on something you're trying to publish--and spend some significant time reworking your draft to incorporate their advice, as well as polish your writing in other ways. Take your time with this; spending a good couple of months working on the paper will both give you plenty of time to really make it shine, as well as demonstrate to your advisor that you're serious about improving it. Once you've made your revisions, get as many professors (or advanced PhD students) to review the new draft as you can, and solicit comments from them. If necessary, take the time to do another round of revisions.

At the end of this process, you should have a really solid piece of writing that you can use in your application, and you'll also have demonstrated to your advisor (and possibly other involved faculty) that you've got the skill and patience to improve a piece of writing significantly. That's a really important skill as a philosopher, as it's exactly what you'll be doing both with your dissertation and with papers you submit to be published. Remember that no one produces perfect drafts of substantive papers right out of the gate--most journal article submissions go through at least two drafts before they're finally accepted for publication, so "revise and resubmit" are words you're going to be hearing a lot if you become an academic. A big part of what separates successful researchers from unsuccessful ones is the ability to stick with a project through multiple drafts and continue to improve and resubmit the same piece until it's the best it can be.

3. Letters of Recommendation

The people you'll have worked with on this process will be in a much better position to write you a strong letter of recommendation honestly stating that you're ready to pursue a PhD. If you've got a decent philosophy GPA, you should also already be familiar enough with some other faculty members that they too can write you letters. Approach them early and ask if they'd be willing to write you a letter; it's never to early to get that stuff on their radar.

4. GRE

Take the GRE seriously, but don't stress over it too much. Most schools use GRE scores only as a kind of "weeding" tool, letting them eliminate applicants who don't achieve a certain (rather low) minimum score right away, reducing the number of applications they need to look at in detail. Once you make that cut-off, your GRE score won't play a significant deciding role in your admission decision. If you're going to study, focus primarily on doing well on the analytic writing section, as that's the part that people tend to care about the most in philosophy. Otherwise, just shoot for decent scores across the board; there's no reason to bend over backward trying to get spectacularly good scores, like you (perhaps) did when taking the SAT as a high school student.

People sometimes get the mistaken idea that it's absolutely essential to get a really, really good GRE score in order to have a shot at top programs, and so end up taking the test over and over again trying to improve their score.

That's usually a waste of time (not to mention miserable, because the GRE is the worst), but it's an understandable misapprehension. Lots of people who choose to pursue a PhD were huge over-achievers in high school and college, and many of them probably took the SAT or ACT multiple times trying to improve their scores. That can be worthwhile for the SAT since there are so many people who get extremely high (or perfect) scores applying to top colleges, but the GRE is different. Not only do scores skew much lower (I've never met someone who got a perfect GRE score across the board: the test is actually pretty hard), but the scores themselves don't play as central a role in the admission decision as they do for undergraduates. With the exception of analytic writing, my GRE scores were not terribly high (I've successfully repressed most of my memory of applying to grad school, but I think my percentile was in the low 70s on quantitative and somewhat higher on verbal) and I ended up at a great program. The writing portion is by far the most important for philosophy, but even that is significantly overshadowed by letters of recommendation and (even more strongly) writing sample quality.

5. Where Should You Apply? How Should You Make Your Final Choice?

Brian Leiter's Philosophical Gourmet is the most well-known ranking of graduate programs in philosophy, so you'll want to take a look at that. I think the sub-specialty rankings are somewhat more meaningful than the overall rankings, but take the whole thing with a grain of salt: there are some serious methodological concerns with how Leiter does his survey and compiles his statistics.

About the best you can say is that there might be some positive correlation between the quality of a department's graduate program and its PGR ranking. The sub-specialty rankings, at least, will tend to reflect the general professional perception of the quality of the research being done on a particular topic at a particular institution; this isn't the only thing that matters for choosing a graduate program, but it is a thing that matters, particularly when it comes to the impact that letters from those people will have on your chances in the job market. My advice, then, is to put somewhat more stock in the rankings for the area you're interested in studying than you do in the overall rankings but to take even those rankings with a hefty pinch of salt. Do your own research, and try to get a holistic impression of your various choices.

More generally, here are some things you might want to consider:

• Average time to matriculation. Does the department have a good track record of helping its students graduate in ~6 years or fewer? If not, how many take longer than that, how much longer than that do they take, why do they take longer, and how are they funded? Is there a high attrition rate (that is, to a lot of people quit or transfer partway through the program)? Why? This can be harder to find out, but you should ask around about it. Departments with very high attrition rates often have serious problems.

• Funding amount and source. It goes without saying that you absolutely should not be paying out of pocket for a philosophy PhD, but in addition to a full tuition waiver (which is mandatory), what does the standard stipend look like? Is there a way to supplement any departmental funding with either teaching or external fellowships? If you don't get supplemental funding, is what the department provides enough to live comfortably on for at least 5 years? Is funding contingent on a large teaching load? For how many years? Are you guaranteed a year or two of funding in which your only real responsibility is to write your dissertation, or are you expected to continue teaching classes while you write? I had many friends at CUNY while I was in grad school, and they were expected to teach several undergraduate courses every semester beginning in their second year. That got them lots of teaching experience, but it also made it much harder for them to progress in their writing; it should be avoided if possible.

• Placement record. How many of the department's graduates in the last ten years have gotten jobs? How many of those jobs are tenure-track jobs at good institutions, and how many are adjunct positions? This information should be somewhere on the departmental website, but if not you can email the department administrator and ask for it. If they're reluctant to provide placement data, that's a big red flag. The best predictor of your job prospects is the success rate of recent graduates from your department.

• How many other graduate students work on your topic or something closely related? This is a balancing concern. If you're the only one interested in your area, it might be hard to have good discussions and develop professional relationships. On the other hand, if there are a dozen other grad students working on exactly the same thing you want to work on, it will be very difficult to distinguish yourself and you might have trouble getting personal attention from the faculty who work on that area.

• Advisor possibilities. Are there several people you can see yourself working closely with? Where are they in their careers? Are they accepting new graduate students? Do they teach graduate classes? Are they good advisors? In general, you should avoid choosing an institution solely on the basis of a single faculty member's presence; you never know if he or she will go somewhere else, take a sabbatical, stop accepting new students, retire, or whatever. Ideally, there should be at least three people you can see yourself working with, and they shouldn't all be either very young or very old.

• Relationships with other institutions. I went to grad school in New York City, and we had what was called a "graduate consortium" that consisted of the philosophy departments of Columbia, NYU, Rutgers, Princeton, The New School, and CUNY. Any graduate student at one of those places could take classes at any of the others for free, and the class would count toward the PhD. This was awesome, and made it much easier to find interesting classes to take. This kind of arrangement exists in other places as well, and is worth looking into. It might be a mitigating factor for a somewhat weaker departmental ranking in your area of interest; if there's another institution nearby with a much stronger ranking and you can take classes there freely, then it doesn't matter as much what your home institution looks like.

• Culture. Talk to the grad students. Do they seem happy? Miserable? Stressed? Do they compete with one another, or are they friendly? Do they hang out socially? Do they seem like they have interests outside philosophy, or are they all workaholics? This stuff might not seem important, but you're going to be spending the next 5-7 years of your life with these people, so if they're all miserable you probably will be too.

• Location and context. Again, this might not seem all that important, but you're going to have to live here for 5+ years. Are you going to hate every second of living in the city? Are there things to do? Places you might enjoy hanging out? How expensive is it to live there? Can you afford a decent place on a grad student's salary? Is there an easy way to relax when you're not working? These things will be important to your mental health, and can really impact how quickly (or even whether) you finish.

Further Reading:

• Thinking About Graduate School in Philosophy?

• The APA Program to Grad School in Philosophy

• “Should I Go to Graduate School in Philosophy?”

• Eric Schwitzgebel's Guide to Applying to Philosophy Grad School

• The Philosophical Gourmet Rankings

Question Sightings: Too numerous to list.

In an effort to keep the quality of answers high here, right now we're limiting contributions to approved contributors. If you're interested in becoming an approved contributor and are a panelist in /r/AskPhilosophy, please see this thread and express your interest.

For the time being, we are limiting this call to panelists with a history of contributions to /r/askphilosophy. We're also limiting this invitation to people who have at least a graduate background in their topic. If there is some special reason why someone who doesn't fit these criteria feel they can still contribute, this can be managed case-by-case. Examples of compelling reasons for inclusion would be an especially notable record as a panelist on /r/askphilosophy, or that they've done one of the Weekly Discussion pieces on /r/philosophy, or some similar contribution.

If quantum mechanics is really an indeterministic theory, might we recover genuine free will by appealing to quantum mechanics? To what extent does quantum mechanical indeterminacy impact the macroscopic world and things like us?

TL;DR: Most of the time, quantum mechanical uncertainty doesn't matter for the macroscopic world. Based on what we know, quantum mechanical superpositions are very fragile in active macroscopic environments, meaning that they rarely last long enough to make a significant impact on classical dynamics or macroscopic systems. Our brains are classical systems, and so exotic QM behavior is unlikely to make a significant difference in our cognition.

There are really three questions here.

• Is quantum mechanics relevant to the question of determinism generally?

• If quantum mechanics is indeterministic, does that have any implications for determinism at the classical level?

• If quantum mechanics is indeterministic, does that have any relevance for free will?

I think the answers to these questions are, respectively: strongly yes, yes with some qualification, and almost certainly no. Here's why.

If the dynamics of quantum mechanics are really genuinely stochastic, then the universe is indeterministic, period. If the same initial state is compatible with multiple future states given the physical laws, then determinism is false, because that's the thesis of determinism. Whether or not QM is stochastic in a deep (i.e. non-epistemic) way is still very much an open question, but if it is then we live in an indeterministic universe, end of story.

There's a separate question about whether or not quantum indeterminism (if it exists) is likely to regularly make a difference to things like us, who mostly live in a medium-sized world inhabited and influenced by medium-sized things. That is, even if we live in an indeterministic universe, does it make sense for us to care about that fact for most purposes? It is not out of the question that this might be the case: we know that sensitive dependence on initial conditions is a real thing, and it's at least possible in principle that in some cases the sorts of changes in initial conditions corresponding to quantum stochasticity might (eventually) have macroscopic consequences, particularly given the fact that entangled QM systems seem to be able to exert a causal influence at space-like separation.

However (and this is the qualification on my "yes" answer), we have fairly good reasons to think that this sort of thing wouldn't happen regularly: that it wouldn't play a central role in the dynamics of things at the classical level. There are two reasons for this. First, we haven't ever detected anything that looks like that sort of effect; classical mechanics appears to be entirely deterministic. This is compatible either with the possibility that QM is deterministic, or that quantum stochasticity generally doesn't propagate into macroscopic behavior. Second (and more compelling), quantum states that aren't "pure" are incredibly fragile. That is, systems in superpositions of observables that are central to the behavior of classical objects (spatial position, momentum, that sort of thing) don't tend to last very long in classical or semi-classical environments (this is part of why quantum computers are so tricky to build). If quantum mechanical stochasticity were to regularly make a difference in the dynamics of quantum systems, particles in states that are balanced between one potentially relevant outcome and another would have to stick around long enough for classical systems to notice and respond.

Based on what we know about how quickly classical environments destroy (i.e. decohere) quantum mixed states, it's unlikely that this is the case. Even very high speed classical dynamics are orders of magnitude slower than the rate at which we should expect quantum effects to disappear in large or noisy systems. Max Tegmark lays all this out very nicely in "The Importance of Quantum Decoherence in Brain Processes".

This, in turn, suggests an answer to the third question: is quantum indeterminism relevant for free will? The answer here, I think, is fairly clearly "no," for reasons related to what I said above in connection with the second question. Even in the brain--a very sensitive, complex, and dynamically active system by classical standards--the time scales of brain process dynamics and decoherence simply don't even come close to matching up. If there is stochasticity at the quantum level, it's coming and going so quickly that your brain never has the chance to notice, and so as far as the brain's dynamics are concerned, quantum mechanics might as well be deterministic.

Even if this were not true--if the brain were somehow special, and sensitively dependent on quantum states in a way that other macroscopic systems aren't--it's not very clear that this would get us much in the way of "free will." Generally, what we want when we want free will is some sense of control or multiple open options that we might choose to take. If there are multiple ways that our brain could evolve, but which of those multiple outcomes actually happens is just a matter of chance, then it's not clear that we're in any better a position than we were in a deterministic universe.

For more information, see Max Tegmark's "The Importance of Quantum Decoherence in Brain Processes", as well as some of the work by W.H. Zurek, especially "Decoherence and the transition from quantum to classical", "Decoherence, Einselection, and the Quantum Origins of the Classical", and "Relative States and the Environment: Einselection, Envariance, Quantum Darwinism, and the Existential Interpretation".

Question sightings: 1, 2, 3, 4, 5, 6, 7

Everett's many-worlds interpretation of quantum mechanics is frequently conflated with David Lewis' modal realism, multiverse theories, or other similar positions. How does Everett's interpretation fit with these other ideas?

TL;DR: Calling Everett's interpretation "many-worlds" is something of a misnomer. The theory itself doesn't posit the existence of multiple worlds or "parallel universes," but rather just the existence of many "branches" of a single world which don't easily interact with one another. The interpretation is strongly distinct from modal realism: in Everett's interpretation, only those outcomes which are consistent with the laws of physics and the history of the actual world (i.e. things that are physically possible) are represented as "branches" in the world. Modal realism, in contrast, maintains that any state of affairs which is logically possible corresponds to a real possible world.

Detailed answer:

Here's how Everett's interpretation works. First, a little set-up. Here's the measurement problem, which is why all of this stuff is necessary in the first place.

Suppose we want to measure the x-axis spin of some electron E which is currently in a y-axis spin eigenstate (that is, it's y-axis spin has a concrete, determinate value). Y-axis spin and x-axis spin are incommensurable properties of an electron (like position and momentum), so the fact that E is in an eigenstate of the y-axis spin observable means that E is also currently in a superposition (with expansion coefficients equal to one-half) of being in x-axis spin “up” and x-axis spin “down.” The "expansion coefficients" just give us the standard QM probabilities, so the fact that we have expansion coefficients that equal 1/2 means that there should be a 1/2 probability that we'll measure x-axis up, and a 1/2 probability that we'll measure x-axis down.

Because quantum mechanics is a linear theory, the superposition of E should "infect" any system whose state ends up depending on E's spin value. So, if nothing strange happens--if the wave function doesn’t collapse onto one or another term--then once we perform our experiment, our measuring device should also be in a superposition: an equally weighted combination of having measured E’s y-axis spin as “up” and having measured E’s y-axis spin as “down.” And if nothing strange continues to happen--if there is still no collapse--then once we’ve looked at the readout of the device we used to measure E’s spin, the state of our brains should also be a superposition (still with expansion coefficients equal to one-half) of a state in which we believe that the readout says “up” and a state in which the readout says “down.”

This is really, deeply, super weird, because it doesn't seem like we ever find our measurement devices in superpositions of different states, and I don't even know what it would be like for my brain to be in a superposition of having observed different experimental outcomes. In every experiment we've ever performed, it seems like we get a concrete outcome, despite the fact that QM says we almost never should. As I said, this is the measurement problem. It's really hard to overemphasize how weird this is, and how straightforwardly it follows from the basics of QM's formalism. Hence all the worry about interpretation of QM.

Collapse theories get around the measurement problem by supposing that at some point, there's a non-linear "correction" to the wave function that "collapses" its value onto one option or the other. However this collapse works, it has to constitute a violation of the Schrodinger equation, since that equation is completely linear. But let's suppose we don't want to add some mysterious new piece of dynamics to our theory. The goal of Everett's interpretation is to explain QM behavior without having to postulate anything new at all; everything that happens is right there in the wave function and the Schrodinger equation (this is enticingly parsimonious).

So, let's suppose that the Schrodinger equation is the complete equation of motion for everything in the world: all physical systems (including electrons, spin measuring devices, and human brains) evolve entirely in accord with the Schrodinger equation at all times, including times when things we call “experiments” and “observations” take place. There are no collapses, no hidden variables, nothing like that. What's left?

The Everett interpretation explains the puzzle of the measurement problem--the puzzle of why experiments seem to have particular outcomes--by asserting that they actually do have outcomes, but that it is wrong to think of them as only having one outcome or another. Rather, what we took to be collapses of the wave function instead represent “branching” or “divergence” events where the universe “splits” into two or more “tracks:” one for each physically possible discrete outcome of the experiment. We end up with one branch of the wave function in which the spin was up, we measured the spin as up, and we believe that the spin was up, and another branch where the spin was down, we measured it down, and we believe it was down.

These branches don't form distinct worlds, but rather just distinct parts of a single wave function whose probability of interacting with one another is so low as to be effectively zero in most cases. Each branch of the wave function then continues to evolve in accord with the Schrodinger equation until another branching event occurs, at which point it then splits into two more non-interacting branches, and so on.

The important point is that these branching events occur whenever the value of some superposed observable becomes correlated with another system. There's nothing special about measurement, and electrons are causing branching events all the time all over the place by interacting with other electrons (and tables and chairs and moons, &c.). Likewise, only those outcomes which are permitted by the Schrodinger equation's evolution of the universal wave function actually end up happening; you don't get a branch in which E had spin up, we measured spin down, and believed it was spin up (despite the fact that such a case is logically possible), since that's not a situation that's permitted by the equation of motion and the initial conditions.

The determinism in this theory is so strong that it doesn't seem to leave any room for ignorance about the future at all. This is not the same sort of lack of future ignorance that we find in, for example, classical determinism; it isn’t just that the outcome of some experiment might in principle be predicted by Laplace’s Demon and his infinite calculation ability. It goes deeper than that: there doesn’t seem to be any room for any uncertainty about the outcome of any sort of quantum mechanical experiment. When we perform an experiment, we know as a matter of absolute fact what sort of outcome will obtain: all the outcomes that are possible. We know, in other words, that there’s no uncertainty about which outcome alone will actually obtain, because no outcome alone does obtain: it isn’t the case that only one of the possibilities actually manifests at the end of the experiments--all of them do.

All of the apparent indeterminacy--the probabilistic nature of QM--is based on the fact that we have no way of telling which branch of the "fork" we'll end up experiencing until the fission event happens. Both outcomes actually happen (deterministically), but I have no idea if my experience will be continuous with the part of me that measures "up" or "down" until after the measurement takes place. That's how the standard probabilistic interpretation of QM is recovered here.

It's interesting to note that two branches of the wave function that have "split" don't stop interacting with each other entirely; the strength of their interaction just becomes very, very small. This suggests that in principle we should be able to set things up such that two branches that have diverged are brought back together, and begin to interfere with one another again. If we could figure out a way to do that, it would serve as an experimental test for the many-worlds interpretation. We haven't figured out how we'd go about doing that even in theory yet, but it is possible in principle--a fact that most people don't realize. This is also part of why the "many worlds" of Everett's interpretation are so distinct from the "possible worlds" of Lewis' modal realism, or even the "parallel universes" of other physical multiverse theories: in addition to the fact that the possible worlds of modal realism correspond to every logically possible state of affairs (while the branches of Everett's interpretation correspond only to the various physically possible outcomes of past quantum mechanical interactions), the "many worlds" of Everett's interpretation lack "causal closure."

When we talk about a "parallel universe" or a "different possible world," we generally assume that each universe (or "world") is causally closed. That is, only things that are a part of some world can have a causal impact on things in that world. If it were possible to travel between two possible worlds, in what sense would they be distinct worlds at all, rather than just different regions of a single world? As soon as causal interaction is on the table, we seem to lose any criterion of demarcation between separate universes or worlds. Because the different wave-function branches created by a divergence event in Everett's interpretation are merely very very unlikely to interact, it's more accurate to think of them as constituting a single world with many different "parts" which, in practice, have very little to do with one another. The fact that it is in principle possible to cause two separated branches to recohere (and thus interact with one another again) is enough to say that, on this theory, there is still just one world.

For more information, see the SEP article on Everett's interpretation, as well as David Albert's Quantum Mechanics and Experience, and Dewitt & Graham's anthology The Many Worlds Interpretation of Quantum Mechanics.

Question sightings: 1, 2, 3, 4, 5