I think this is a faithful rendering of Kripke’s argument against mind-brain identity:
P1. For any two rigid designators A and B, if A and B are coextensive, then the statement “A is B” will be a necessary truth. (Kripkean Assumption)
P2. If identity-physicalism is true, the names “C-Fiber Firing” and “Pain” are both rigid designators and coextensive.
C1. Therefore, If identity-physicalism is true, “C-Fiber Firing is Pain” is a necessary truth. (By P1 & P2)
P3. “C-Fiber Firing is Pain” is not a necessary truth.
C2. Therefore, identity-physicalism is false. (By P1, P2, C1 & P3)
The key premise here is (P3). It rests on the idea that conceivability is a guide to metaphysical possibility. I can perfectly conceive of a world in which there is pain, but not a single C-Fiber firing. If that’s true, and pain is possible without C-fiber firing, then it’s not a necessary truth. But the identity theory implies that it would be, and thus is false.
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ReplyDeleteOf course, the intuition required for (P3) might have the following deficiency:
ReplyDeleteThere are many cases in which, prima facie, it would seem that a given identity statement is not necessary, but we eventually discover that it is. A classic example might be our concieving of a world in which some chemical element called H2O is not identical to water, before we actually discovered that it is. Once we discovered that it is a necessary truth, that concievability seems to dissipate very quickly. This seems to be an issue with instances of a posteriori necessity in general.