Some Thoughts on Kripke's Argument Against Identity Theory

In an excerpt from Naming and Necessity, Kripke argues that the type-type identity theory, which states that for every brain state there is corresponding mental state which is identical to that brain state, is rendered absurd using modal metaphysical notions, particularly the notion of rigid designation. Kripke explains that a rigid designator “designates the same object” “in every possible world” where that object exists (2). He then goes on to state that if there are two rigid designators, such as “‘R1’” and “‘R2’”, which are in fact identical to each other, as represented by “‘R1=R2’”, then, if it is true that “‘R1=R2’”, it is necessarily true that “‘R1=R2’” (9). This is because truths about identity are necessary truths. He then applies this to the type-type identity theory. He states that on this view, brain states are identical to mental states and so they necessarily share the same properties. Kripke uses the example of  “C-fiber stimulation” to illustrate how this, for him, leads to an absurdity within identity theory (12). He says that since the C-fiber stimulation is a physical state, this will ultimately be identical to the mental state of being in pain. Moreover, since the C-fiber stimulation will be identical to the mental state of being in pain, and given that if it is true that “‘R1=R2’”, it is necessarily true that “‘R1=R2’”, the identity theorist will be committed to saying that it is impossible that there be pains without C-fiber stimulation and, conversely, it will be impossible for there to be C-fiber stimulation without there being some pain (9). Kripke takes this consequence to be “surprising and counterintuitive,” and thinks that the intuition that the correlation of the sensation of pain and C-fiber stimulation is contingent and he thinks that this intuition is not likely to be defeated by the identity theorist’s proposal (12).

I think this argument is very interesting, but I also think that Kripke could have gone further. It seems that he is right that if type-type identity theory is true, then the facts about mental and physical states must, necessarily, be the same. This, I think, leads to a severe problem for the identity theorist. It is important to note that when the identity theorist identifies mental states with physical states he reduces the mental states to physical states. Now, it is commonly held that the mental is qualitative in nature, while the physical is quantitative. However, if the identity theorist attempts to reduce mental states to physical states, he will not preserve the qualitative nature of the mental. Rather, he will be eliminating the mental since the reduction, for him, goes from mental to physical, but the physical is quantitative not qualitative. So, it seems that using Kripke’s point, that if identity theory is true then mental states and physical states must be the same, results in the absurd elimination of the mental on identity theory.

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