Proof against physicalism, slightly elaborated but basically the same as before.
Section 1, demonstration that the objects of perception cannot also be external objects:
This is taken from the argument from hallucination against direct realism. If you have a brain in a vat which is hooked up to a computer and being fed signals simulating those which occur during normal perception, then there will be subjective experiences associated with this brain. Some of those experiences will be experiences of objects in a spatial world. Let’s say the BIV Is being fed a simulated version of sitting in front of a mirror with the top half of its skull removed, so it can see a brain. THAT brain cannot possibly be the same as the “real brain” that is the cause of the experiences. Why? Because the “real brain” is in a vat.
Section 2, main body of proof:
Definitions:
Physicalism: The claim that the only reality is physical reality.
P1) Your experiences of objects (like chairs/brains) ("subjective experiences") P1 refers to the brain in the mirror from the above example.
P2) The external things which cause you to have experiences of objects ("external stimuli"). P2 refers to the brain in the vat from the above example.
Premise (A): P1 and P2 are not the same thing and should not be confused. The brain in the mirror cannot be the brain in the vat.
Premise (B) : P1 and P2 account for everything which exists.
The proof now rests on the potential ways of defining "physical" with respect to P1 and P2.
(C) Physical is (P1):
If "physical" is defined as (P1) then there is a reality which isn't physical - the world of causes - the world of P2.
Conclusion: Physicalism is false.
(D) Physical is (P2):
If “physical” is defined as (P2) then there is a reality which isn’t physical – the world of experiences – the world of P1 – otherwise known as the mental realm.
Conclusion: Physicalism is false.
(E) Physical is both (P1 and P2):
Since this is the option people challenged before, it is going to need further explanation. The defence which was offered was as follows:
Premise: physicalism is true
Therefore, both P1 and P2 must be physical.
Conclusion : Physicalism is true.
What’s wrong with this?
First problem : It’s completely circular. It assumes its conclusion.
Second problem : It leads to the logical conclusion that there are two physical brains:
Even if you are going to claim that P1 and P2 are both, in some way, “physical”, you cannot claim that both of them are “physical objects”. If you do that then you have got two different physical brains in the example of the brain in the vat. One of those physical brains is in the vat and one of those physical brains is sat in front of a mirror. But if there are two physical brains, shouldn’t there be two sets of experiences? How can you fix this? It looks like you are going to have to define one of the brains as not being physical. This is the only way to avoid the conclusion that there are two physical brains – one in a vat and one in front of a mirror. But doing this is a direct contradiction of the claim that both P1 and P2 are physical. I need a term for whichever brain is not a physical brain.
Conclusion (just as before): Physicalism can’t even be specified without a contradiction and is therefore false.
(F) Physical cannot be defined in terms of (P1) and (P2):
Combined with premise (B), we are now saying that amongst the entire class of things which exist there is nothing we can meaningfully define as “physical”.
Conclusion: “Physical” doesn’t refer to anything at all, physicalism is false.
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We have now explored all the logically possible ways of defining “physical”, given P1 and P2. All of them lead to the conclusion that physicalism is false. And we haven't even mentioned "brain processes", so they are irrelevant to the proof. Nothing you can say about them makes any difference to THIS proof. The only way to escape it is to argue for eliminativism, and this would take the form of denying that P1 refers to anything at all (NOT that P1 is the same as P2, because that would be (E) and it's incoherent, as the eliminativists know only too well).
