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The SEP article on Zombies writes:

Zombies in philosophy are imaginary creatures used to illuminate problems about consciousness and its relation to the physical world. Unlike those in films or witchraft, they are exactly like us in all physical respects but without conscious experiences: by definition there is ‘nothing it is like’ to be a zombie. Yet zombies behave just like us, and some even spend a lot of time discussing consciousness.

Few people think zombies actually exist. But many hold they are at least conceivable, and some that they are possible. It is argued that if zombies are so much as a bare possibility, then physicalism is false and some kind of dualism is true. For many philosophers that is the chief importance of the zombie idea.

Why is it true that if zombies are possible that physicalism must be false, and which—if any—philosopher's have actually contended that zombies are possible, rather than merely conceivable in a vague sense?

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    Because I am a physicalist, I agree that if these zombies are exactly like us in all physical respects including all internal materials such as neurons etc, then they would have to be conscious like us and therefore they are impossible. However, I could easily imagine zombies that are externally exactly like us but whose brain has been completely replaced by something that simulates the external behavior of a human but would not be conscious. I would guess this kind of zombie is of no interest to philosophers. Is it?
    – FrankH
    Nov 4 '11 at 23:56
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    @FrankH: You can't imagine either--- you are deluding yourself. Zombies are stupid, and a simulated brain is as good as the real thing, since simulated computation is still computation of the same kind. But it is important to note that the whole question is basically an attack on logical positivism--- it is asking you to think about something which has no empirical determinant and pretend it is meaningful. It's the modern angels dancing on a head of a pin, and it is a terrible, terrible abuse of the positivists.
    – Ron Maimon
    Apr 2 '12 at 6:17
  • @RonMaimon, you may be right that I am deluding myself. But what I kind of imagined is that instead of computation to decide on the next output given the current state and all inputs, there could possibly be a HUGE lookup table that decided on the next output given the current state and all inputs. So whatever computational process gives rise to "consciousness" wouldn't really exist here since there is no computation - only table lookup... I don't know for sure that there would NOT be consciousness in the lookup table, but I somehow doubt it would result in consciousness.
    – FrankH
    Apr 4 '12 at 1:09
  • If zombies were possible, and they were exactly like humans in every observable aspect, how could we find out they are zombies? The question doesn't make sense. Physicalism is not refutable.
    – Trylks
    Aug 25 '13 at 23:41
  • @Trylks - I am a physicalist, don't get me wrong, but only because alternative theories are less likely to reflect reality ("the way things are"). If you think physicalism is 100% irrefutable, however, you are ignoring a rich body of literature in Phil of Mind. Check out SEP or Wikipedia, or even just google "refutations to physicalism", there are interesting ideas there which ought to make you question your certainty. :P
    – stoicfury
    Aug 26 '13 at 5:06
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The key phrase is exactly like is in all physical respects. Physicalism states that if X and Y are physically identical, X and Y are identical. It doesn't matter for the argument that it happens to be zombies or has something to do with consciousness; it just states: imagine that X and Y are physically identical but X and Y are not identical. Of course we have just imagined non-physicalism!

The interest, if there is any, in this form of argumentation is that there is no clear causal mechanism by which physical configuration is known to generate consciousness. We also don't have a clear causal mechanism by which people laugh at the antics of Bart Simpson, but for some reason Bart-Simpson-grumps have not made it into philosophical discourse.

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    This may or may not be on topic here, but how this line of argumentation is not question begging remains to me a mystery. Premise 1: Physicalism is true. Premise 2: Zombies possess the same physical states as non-zombies. Conclusion: Physicalism is false. Without significant argumentation from the non-physicalist, there is no principled reason one should accept both Premise 1 and Premise 2. Further, an equally valid conclusion to be drawn from this line of argumentation would be "Zombies do not possess the same physical states as non-zombies."
    – Jaime Ravenet
    Nov 3 '11 at 3:21
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    @Jaime Ravenet - To me it seems just a restatement of the distinction between physicalism and not-physicalism. It doesn't settle anything or even provide any insight.
    – Rex Kerr
    Nov 3 '11 at 5:19
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    @JaimeRavenet: The point is, as you indicate, it is impossible simultaneously hold both premises-- so, either physicalism can hold, or Zombies (as defined) are possible, but not both. That's the point of the argument. If Zombies do not possess the same physical states as non-Zombies, then they are not Zombies; their possessing the same physical states is the whole point of the thought experiment.
    – Michael Dorfman
    Nov 3 '11 at 8:08
  • Nor is there any evidence of any non-physical causal mechanism for consciousness. For all we know, rocks ARE conscious in some fashion.
    – Shufflepants
    Oct 5 '16 at 14:56
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The standard argument is a reductio: assume that physicalism holds; therefore, consciousness is strictly reducible to physical states; Zombies possess exactly the same physical states as humans but lack consciousness; therefore, physicalism cannot hold.

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Regarding the truth of physicalism WRT the existence of Pzombies, in order to reach the conclusion that "physicalism is false" from the premises given, then it must be part of the definition of "physicalism" that all mental states are identical with and reducible to physical states. It's this necessary property implicit in the definition of the term that would make physicalism false (if you buy such arguments) because the Pzombie counts as a conceivably possible counterexample to the universal claims of the physicalist. However, if one offered a more nuanced definition for physicalism, something like "To have mental state X is to be in physical state Y" such that "physical state Y" is functionally defined, then non-physicalist arguments for and from Pzombies become significantly more difficult to maintain.

One philosopher who argues for at least the possible existence of Pzombies is (famously) Chalmers.

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This just seems a big case of how you feel like defining 'zombies' and 'physicalism' to get the result you want, and has little to do with the commonly accepted cultural artifact of the Halloween/fright movie concept.

Sort of like how there are all sorts of proofs of the existence of deities, but in the end those proofs talk about idealized logical ideas and have little relation to the Abrahamic God or Zeus or whatever.

Or, to take the definitions seriously, one could consider the definition of 'zombie' as posing a possibility for which it is impossible to test. That is, it is not a scientific concept, one that is distinguishable or testable in the real world. If the external activities of a conscious person are indistinguishable from one that lacks consciousness, that means there is no way to test if a person is conscious or not so you can't know if a particular individual is a zombie. So it can only be a thought experiment and not a real one.

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