Article Summary: “An Argument for the Identity Theory” by David Lewis


David Lewis presents an argument for the identity theory of mind – the view that mental states (Lewis here calls them ‘experiences,’ but I shall call them mental states since that is a more common term) are equivalent (in the strict sense of identity) to neural states. His argument deploys the functional understanding of mental states, which says that such states are characterized by the causal roles they fill. Lewis then advocates for the explanatory adequacy of physics – the view that physics can explain all of the causal relations of physical phenomena. Since mental states cause physical phenomena, and only physical things are needed to explain such a causal relation, mental states just are physical phenomena causing other physical phenomena (e.g., behavior). In simpler terms, take a mental state M. M fills causal role R. Due to the explanatory adequacy of physics, only a physical state P can fill R. Therefore, M = P.

Lewis begins by sketching his theory in relation to an example of a lock. The state of being unlocked is at first characterized in functional terms. That is, ‘being unlocked’ is a functional state. Then, with regard to a specific lock, we can see just what it is which fulfills that functional state. And in the case of, say, a cylindrical combination lock for bicycle chains, the functional state of ‘being unlocked’ will be found to be a proper alignment of slotted discs (a purely physical state). Lewis wants to say that the same is true of the mental states which fill functional states in theories of mind.

Lewis then clarifies the nature of the identity theory of mind in general, defending it against objection. The first point Lewis makes is that the identity theory intends for certain physical states to be mental states, but not that the physical state is the object of the experience. So the physical state of seeing red is not itself red. Lewis then diffuses the objection that since mental states are by analytic necessity unlocated, and physical states meanwhile are located, that mental states cannot be physical states. Lewis thinks this is an unwarranted objection because he sees no basis for claiming any form of necessity for the claim that that mental states are unlocated.

Lewis then proceeds to objections relating to the difference between neural-state and mental state-ascriptions. Identity theory claims that the two sorts of ascriptions refer to the same underlying phenomena, not that they do so in the same sense. So just because in some instances the truth value of the two sorts of ascription are different, the identity theory is not necessarily false. The two sorts of ascription can refer to the same phenomena, albeit in difference sense.

Additionally, for Lewis it is not the case that the identity theory is false just because mental-state and neural-state ascriptions are not synonymous, while the identity of attributes predicated by such ascriptions is established via such synonymy. For ‘having‘ an experience is being in a definitive state which fulfills a specific causal role, whereas having the attribute predicated of someone when it is said they are having the experience is the attribute of being in that state.

Lewis then presents the first premise of his argument: mental states’ defining characteristics are their causal roles. Lewis says that a mental state is in fact the causal role that it fills. He says that this notion is an expansion of the ‘topic-neutral’ analyses of mental states presented by JJC Smart; the only difference is that Lewis makes explicit the causal connections of the state. Such a theory is in opposition to both epiphenomenalism and behaviorism because it explicitly holds that mental states are causally efficacious, unlike these theories.

Lewis characterizes this view of mental states as a successor theory to behaviorism: like behaviorism, Lewis’s theory recognizes that the causal roles of a mental state are analytic; unlike behaviorism, his theory allows for mental states to be causes and effects, allows for the interdefinition of mental states in terms of each other, and can handle exceptions. Behaviorism could not handle an exception because it insisted that mental states were mere behavioral dispositions. Yet the complete paralytic could still be in pain despite not being able to exhibit any behavior. Lewis’s theory skirts the issue of exceptions by characterizing mental states as the typical (rather than exceptionless) occupants of specific causal roles.

Lewis concludes his defense of his first premise by saying that he is relying upon the analytic statements of mental states inherited from behaviorism when he says that such states are characterized primarily by causal role occupation. It is these causal-centric statements that he is utilizing, all the while avoiding the pitfalls of behaviorism.

Lewis’s second premise is that physics can explain all physical phenomena. That is to say, when a phenomena occurs in, say, a special science such as cognitive science, that phenomena can be explained in terms of a more fundamental science, and in turn that explanation can be explained in terms of a still more fundamental science, until the phenomena is explainable purely in terms of fundamental physics. As such, all causal occurrences can be explained in physical terms. Lewis does not mean to say that necessarily no non-physical phenomena exist. Rather, he thinks that physical states can explain all physical phenomena. If non-physical phenomena exist, they are explanatorily (causally) superfluous. Lewis’s justification for this assertion is that it is a working hypothesis of natural scientists.

It follows from the two premises of Lewis’s argument that mental states just are physical states. For mental states occupy causal roles, and according to the explanatory adequacy of physics, only physical states fill causal roles. As such mental states just are physical states. Further, it is very likely on this view that the specific sort of physical states that mental states are are neural states. The argument for identity theory is thus complete.

Lewis concludes by tackling a potential epiphenomenalist alternative which is consistent with his premises. This view would hold that mental states are non-physical correlates of physical states which are causally efficacious just like the underlying physical states since they are perfectly correlated with them. Lewis denies that such states would be causally efficacious because again, due to the adequacy of physics, there is just no need to posit such a non-physical causal force. Further, even if the theory were correct, it would actually implicate such mental states as duplicates of the physical mental states, rather than as mere correlates of them. And this is a very different position than what the epiphenomenalist wants to argue for.

Logical Outline

Argument Outline – Mental States Equal Physical States

  1. For any given mental state M, M fills causal role R.

  2. Because of the explanatory adequacy of physics, only a physical state P could possibly fill causal role R.

  3. Therefore, M = P.

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