The mind brain identity thesis holds, minimally, that types of sensory states are identical with types of physical states. Saul Kripke contends, however, that such an identity is not the case—based on logical modality. It is my contention that his theory is not conclusive and that Christopher Hill’s criticisms against Kripke’s argument are decisive. The following is an analysis of the argument and Hill’s criticism.

Kripke’s argument against the mind brain identity theory

Kripke first establishes the claim that sensory states and physical states are described using rigid designators, which is any term that refers to the same thing in every possible world in which it exists. Specific examples of rigid designators are proper names and natural kind terms such as heat, water, pain, etc. Kripke notes that the identity put forth in the mind brain identity theory consists of rigid designators. Thus, if the identity of sensory states and physical brain states is true, then it is necessarily true. Likewise, if we can imagine, claims Kripke, a possible separation of sensory states and physical brains states, then their separateness is necessarily true. That is, if we can imagine being in a sensory state apart from being in a physical state, as Kripke claims we can, then the two states are not identical. Therefore, he concludes, the mind brain identity thesis is false and the mind brain duality thesis is necessarily true. His conclusion is reasonable if you grant him one stipulation: All along Kripke maintains that there is no good reason to believe that what is imaginable is not possible—with respect to imagining being in a sensory state apart from being in a physical state [and with rigid designators in general]. If you grant him this position then his argument is sound and we must conclude that the mind brain duality thesis is valid. However, we need not agree with Kripke if we can demonstrate how it is that we might imagine the possibility of something like mind brain dualism, and yet conclude that it is not possible—such is the Achilles heel of Kripke’s position.

Examples of identities

We can say that the first Post Master General is the inventor of bifocals. Such an identity contains the non-rigid designator ‘the inventor of bifocals’ and is therefore, not of the same nature of the identity between brain states and sensory [or psychological] states. On par with the latter identity is the identity of water with h20. ‘Water’ is a rigid designator, as is ‘h20.’ Because of the truth of their identity in this world, it is impossible to even imagine water existing without h20 also existing—such is the modality of rigid designators. Kripke employs the following example: “Let ‘Descartes’ be a name, or rigid designator, of a certain person, and let ‘B’ be a rigid designator of his body. Then if Descartes were indeed identical to B, the supposed identity, being an identity between two rigid designators, would be necessary” (p.329). Kripke maintains that it would “plainly inadmissible” to agree on the one hand, that it is possible [conceivable] for Descartes, a person or mind, to exist without his body, or vise versa, and then conclude, on the other hand, that Descartes is not distinct from his body.

The Achilles Heel

It is the task of those opposed to Kripke’s position to explain how it is that one can imagine the exclusive independence of two symbiotic rigid designators without maintaining the veritable truth of their exclusive independence. Christopher Hill puts forth such an argument. He notes, along with Thomas Nagel, that the imagination consists of perceptual, sympathetic, and symbolic representation. Imagining something perceptually is to induce a mental state similar to the mental state of actual perception—to induce a feeling of perceptual representation such as seeing red, or hearing a train, etc. If you want to imagine seeing red, you imagine what it would be like if you were actually seeing red. Imagining something sympathetically is to induce the mental state of the thing itself—to imagine what it would be like to for somebody else to be in a certain mental state. Essentially, sympathetic imagination is employed for imagining mental states, while perceptual imagination is reserved for imagining brain processes. Hill writes,

“It is also part of Nagel’s story that we can use the perceptual imagination to form images of the absence of brain processes, that is, images that represent situations in which no brain processes exist. Nagel uses this story as the foundation of an account of how we are able to imagine disembodiment. According to Nagel’s account, when we imagine a mental state that is not accompanied by a brain process, what we do is splice together a sympathetic image of a situation that contains a mental state and a perceptual image of a situation in which no brain process is present. The fact that we do this without incoherence has no tendency to show that it is objectively possible for the imagined mental state to exist without being accompanied by a brain process. Rather our ability to do it is due to the fact that there are two types of imagination that operate independently of one another” (p.337).

Essentially, we can imagine the exclusive independence of symbiotic rigid designators such as being in a sensory state and being in a physical state because of the fact that our perceptual imagination and sympathetic imagination work independently of one another. So we can imagine heat apart from molecular motion, water apart from h20, and Descartes, as a mind, apart from his body. We imagine heat, water, and Descartes’ mind, as perceptual representations; and we imagine the movement of molecules, h20, and Descartes’ body, as sympathetic representations.

We can therefore conclude that the contents of our imagination are not a valid indicator of determining whether or not two entities are identical or not. Once again we are reminded that while logic may be useful for consistency and validity, it cannot be employed as a conclusive means to reaching the truth.