Bad Argument: Physicalism Can't Be True

Victor Reppert has come up with an argument that supposedly proves his contention that mind must precede the physical.  According to him, this argument does not rest on any assumption of the primacy of mind, which is the metaphysical notion that mental phenomena, such as rational thought or morality, can't possibly arise from any purely physical source, and therefore mind must exist at the most fundamental level of reality.  In fact, most people who hold this belief are theists who think that physical reality itself is the product of a mind.  This stands in stark contrast to physicalism, which is the metaphysical notion that physical reality is all there is in our world, and therefore any mental phenomena that exist must be a product of that physical reality.  While Victor's argument assumes neither of these metaphysical positions, it still contains a serious logical fallacy  Here it is, in its entirety:

My argument is an attempt to show, not assume, that minds exist first, on the grounds that if they don't exist first, they cannot emerge. Mental states have to be a complexity-fact about the physical world if physicalism is true. But let's take the claim that "I am Victor Reppert" and the claim "I am Hugo Pelland." It seems perfectly conceivable that there is a world physically identical to this one in which you are me and I am you. If you say that such a world is impossible, you need to prove it, since it is conceivable. There is nothing about the physical world that guarantees that I will be me and you will be you. So physicalism cannot be true. - Reppert

The first thing to note about this is that it equates conceivability with possibility, but doesn't distinguish between logical, physical, and metaphysical possibility.  The difference between metaphysical and physical possibility is that physical possibility is bound by that laws of nature as it exists in our world, while metaphysical possibility is a superset of that which relates to what might exist in some possible world (and logical possibility is a superset of both*).  The thrust of Victor's argument is that if it is possible that mental phenomena are not bound by physical reality, then physicalism can't be true.  Let me try to put it in the form of a syllogism:

1. Self-identity, as in "I am Victor Reppert", is a mental state.
2. Victor can conceive of some possible world that is physically identical to ours, but the mental states are different.
2a. This would be the case where Victor's body has the self-identity of Hugo Pelland, and vice versa.
3. Since he can conceive it, there must be such a possible world - in other words, it is possible that different mental states could occur from the identical physical reality (from 1 and 2).
4. If it is possible that different mental states could occur from the identical physical reality, then physicalism is false.
5. It is possible that different mental states could occur from the identical physical reality (from 3).
6. Therefore, physicalism is false (from 4 and 5).

We can see from statement 3 that Victor is using metaphysical possibility in this syllogism, because he's talking about some conceivable possible world - not the world we live in.  We can then assume that use of the word 'possible' in statements 4 and 5 refers to metaphysical possibility.  But here we run into a problem.  Physicalism would only be false in some possible world where different mental states could result from identical physical states.  If some other possible world exists where that is not the case (ie, identical physical states always produce identical mental states), then physicalism could still be true in that world.

It seems that Victor is equivocating on the meaning of 'possible' (and as a trained philosopher, he should know better), because his conclusion appears to say that physicalism must be false in our world.  In order to eliminate this equivocation, we should replace "it is possible" with "it is possible in some conceivable world" (lines 4 and 5), and we should replace "is false" with "is false in some conceivable world" (lines 4 and 6).  It then becomes clear that the implied statement "physicalism is false in our world" doesn't follow from this argument.  The only thing Victor has shown is that one can conceive of a world where physicalism is false.  But that's not the case in our world.

Victor's statement "There is nothing about the physical world that guarantees that I will be me and you will be you" is not true in this world, because our self-identity is a product of our physical experience.  No two brains can be identical because they would need to have all the same experiences of body, time, and place.  It is physically impossible for two different brains to have an identical set of experiences, and since those experiences have a physical impact on the brain (by modifying neural connections), then each brain must have a unique self-identity that is a direct consequence of its unique experience in the world.  In this world, physical reality guarantees that "I will be me and you will be you".  In this world, there is still no reason to assume that physicalism is false, and Victor's bad argument has done nothing to change that.




*Note: I disagree with this standard model of modality, because I think that logic itself is a manifestation of the physical reality of this world.  If there exists some other world that has different (or perhaps no) laws of nature, then it is not necessarily the case that logic in that world would be the same as our logic, but would instead be a manifestation of that reality, whatever it might be.  The philosophical presumption that our world's logic must hold true in all possible worlds is based on anthropocentric bias.  See my article Where Does logic Come from?