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In Lecture III of Naming and Necessity, Kripke presents an anti-materialist argument that directly challenges the mind-brain identity theorists’ physicalism. Kripke is not concerned with any other form of physicalism (e.g., view on propositional attitudes) and does not believe that they will be successful in refuting any parts of his argument1. Kripke challenges the identity theorist to explain how a semantic stable necessity (i.e., it refers to the same thing across all possible worlds) can appear contingently true (i.e., for it to simultaneously be true yet possible false) to someone who already believes that it is true. Only an identity theorist would believe that the identity is true, therefore the argument is directed toward the identity theorist’s view. If water is semantically unstable, it would refer to different things depending on how the world turned out to be, but if the reference is fixed as ‘that stuff’, whatever it may be, that is odorless, falls from the sky, and nourishes me, then water is still semantically unstable.

The challenge that Kripke presents is specifically geared toward identity theorists. Only an identity theorist would believe that pain is identical to c-fibers firing, and if this claim is true, it is necessary. The identity statement appears contingent because we can imagine a world in which one exists without the other. If this appearance can be explained away, then the mind-brain identity is possibly false. The appearance of contingency cannot be explained away by identity theorists. Therefore, the identity is not a necessary one and is therefore not true either

Within this exposition, I will argue that Papineau’s interpretation of Kripke’s anti-materialist argument in Naming and Necessity is correct: it is ad hominem to the mind-brain identity theorist. I will first give quotes from Naming and Necessity to prove my point in that it was directed towards an identity theorist. Second, I will explain how the appearance of contingency arises through necessary a posteriori truths. Finally, I will explain how physicalists can respond to Kripke’s argument.

Kripke presented his argument to the identity theorists very explicitly and repetitively. Identity theorists are called out for the one’s specifically believing some identity, such as c-fiber stimulation and pain. Here are four quotes calling out the identity theorist specifically:

At the beginning of Lecture III in Naming and Necessity, Kripke states that he will conclude with remarks on the mind-brain identity thesis6 after he provides examples showing that necessary a posteriori truths (e.g., this table is made of wood) are possible, as well as contingent a priori truths (e.g., the standard meter in Paris is one meter long). If there is a necessary a posteriori truth, it is usually masked by an illusion of contingency, which is saying that something is in fact possible when in all actuality, it is not. This illusion or appearance of contingency can only be explained away if there is some term in the relationship that selects its referent by a contingent description7. The intuition that the thing that A rigidly designates can exist without the thing that B rigidly designates must be deemed an illusion and therefore be capable of being explained away8. Due to the contingency of some things, A’s rigid designator can exist without B’s actually existing and vice versa. An example of this is “water is H2O”. This statement appears to be contingent, since we can do a thought experiment on a counterfactual in which water turned out to be something else other than H2O. But what Kripke has to say about this thought experiment is that whenever we stipulate this possible world, we are not actually talking about water as we refer to it on the actual world—we are referring to something that looks the exact same as water, taste like water, nourishes us like water, and the rest.

The same can go for the case of heats being identical to molecular motion. Could the movement of molecules have existed in a stipulated world without heat? Yes, in this possible world, the movement of molecules could do everything they do in this world, but instead, in this other world it is not felt as the sensation of heat. This is because the reference of “heat” is fixed by the feeling we get from it. In this case, we are tricked by the appearance of contingency and we mistake the claim ‘heat = molecular motion’ as ‘the cause of our feeling of heat = molecular motion’9. Something to remember, to help avoid confusion, is that if the truth is necessary—it doesn’t matter a priori or a posteriori—it could not have turned out any differently.

Kripke believes that theoretical identities generally involve two rigid designators and are what science aims to discover, making them necessary a posteriori10. One of his main objectives is to show that theoretical identity sentences are necessary, if true (e.g., water is H2O). Theoretical notions in science are fixed by descriptions because they hold some relationship with the device that is measuring it, as well as the humans observing it.

Towards the end of Lecture III, Kripke begins to focus on the on the mind-body problem and uses the examples he has been building up (e.g., necessary a posteriori, rigid designators, the necessary relation of identity, etc.) to present his anti-materialist argument in the fullest form. By using these examples and concepts, Kripke is able to identify three branches of identity theory which he claims to all be incorrect: the event or state of having a sensation (i.e., having mental properties); the pain I felt at noon was my c-fibers firing (i.e., the mental property itself), and pain being the stimulation of c-fibers (i.e., tokens of the mental property). After differentiating types of identity theories, Kripke presents arguments against all three of them and moves on to type-type identities (e.g., pain = c-fiber stimulation)—the identity he wishes to give the most amount of attention to. The type-type identities involving phenomenal concepts should be relatively similar to the scientific type-type identities of heat and molecular motion, along with water and H2O11. Given that we do not think of phenomenal pain as having a contingent reference-fixer, if a phenomenal identity claim seems like it is mostly likely false, then you should think that it actually is false. The same goes for a phenomenal mind-brain identity claim—it cannot be a posteriori necessary. The reason why is because a posteriori necessary is usually due to ‘semantic instability’, but phenomenal concepts are semantically stable.12 There is no room for the phenomenal concept of ‘pain’ to pick out different things, depending on the facts of the world. Pain’s referent does not work like heat, getting to pick whatever thing turns out to be responsible for that certain appearance. Although, the phenomenal concept of pain will refer to the same entity—that hurty feeling within—no matter how the facts are determined.13

Heat and the motion of molecules, as well as pain and c-fibers firing are contingent type-type identity theories. The illusion of contingency only appears if the claim ‘molecular motion might not cause heat sensations’ is true14. The terms ‘heat’ and ‘molecular motion’ are rigid designators, resulting in the identity relation between the two as a necessary one. Now for pain— ‘pain’ is a rigid designator of the phenomenal feel it designates. Now also suppose that c-fiber is a rigid designator—not c-fiber stimulation. However, it is not the case that ‘c-fiber’ is a rigid designator, and it can be replaced with something that is a rigid designator. Because c-fiber is not a part of the ‘pain = c-fiber stimulation’ equation, it is still a necessary relationship. Heat is a three-term relation involving the thing causing the heat, the observer who feels the heat, and then heat itself. With pain and c-fiber stimulation, it is a two-term relation; there is the thing causing the pain, there is the observer who feels the pain, and there is the pain itself—but the observer who feels the pain and the pain itself collapse into one term. Pains cannot be separated from their appearances in the fashion that water and heat are able to15.

Brain states can be imagined sympathetically or perceptually. To imagine what you yourself feel like whenever that brain state is present (e.g., c-fibers firing) is to sympathetically imagine the brain state. To perceptually imagine a brain state is what it is like to observe this brain state from an outside perspective (e.g., an fMRI scan). The two ways of imagining a brain state mean that the identity between pain and c-fiber stimulation is a posteriori because one could observe an fMRI scan of a brain and know that it is the result of c-fiber stimulation, as well as feel the sensation of pain—but not know that the fMRI scan and their sensation of pain are the same exact thing. This would be a lack of knowledge on the individual’s part, and once they gain empirical evidence that it was the case, they would no longer think that they were different. Although, it is still possible for an individual to imagine a possible world in which pain is present without the stimulation of c-fibers or vice versa. This conceptualization would not grant an appearance of contingency to the person who just became aware that they were the same thing, because there is no reason for them to believe that pain’s being identical to the stimulation of c-fibers could have turned out any other way.

Mind-brain assertions of identity lack a contingent reference-fixer and do not choose their referents indirectly through definite or cluster descriptions. Moreover, if one were to conceive of a world in which c-fibers firing doesn’t appear sympathetically (i.e., you yourself experiencing pain or another), the person is not stipulating the fact that c-fiber stimulation (i.e., pain) might not have some feature that is contingent that it is actually displaying—instead, they are supposing that c-fiber firings might be present without pain. By doing this does not explain how physicalists can think that c-fiber stimulation and pain could separate, since it presupposes that the pain and c-fiber stimulation are two distinct states—separate from one another—which is contrary to their outlook. Once the belief that pain is identical to c-fiber stimulation is acquired, there is no room for any further thought that some pain-seeming or pain-appearing state might not be the stimulation of c-fibers. All this would be is the the thought that pains might not be the c-fibers firing—which is the belief that the identity theorist rules out.

Kripke’s anti-materialist argument is presented in a way that allows mind-brain identities to appear as if they can be separated. Identity theorists will say that these identities should not appear in a way they could be possibly separated; but they do appear in such a way; therefore, physicalism is false. This arguments conclusion does is lead a physicalist to the metaphysical conclusion that mind-body do not stand in an identical relation—instead, it supports the idea that it is not intuitive to believe mind-brain identities.

Physicalists are left with a possibility of explaining the consistent appearance of possible falsity (i.e., contingency) by explaining on some level that they do not fully believe the mind-brain identities posited, or any for that matter. And this is why they appear possibly false. Physicalists will still be able to hold on to their belief at a theoretical level, such as the evidence for a range of claims like pains and c-fiber stimulation is sufficient for belief. Physicalists can also allow that this evidence does not give them an intuitive edge to the identities, and that persistent appearance of possible falsity is merely arising from their intuitive feeling that these beliefs they hold are actually false.

Papineau, D. (2007). Kripke’s Proof is Ad Hominem and Not Two-Dimensional. pp. 475-494