David Lewis is well-known for defending a certain version of the identity theory of mind. According to this theory, every mental state is identical with some physical state of the brain. Lewis understood the identity theory of mind along functionalist lines. Mental states are specified functionally, which is to say mental states are defined in terms of input and output clauses. The mental state of pain is functionally characterised as that which is caused by damage to one’s body (usually) and that which typically manifests pain behaviour. The upshot is that the defining feature of a mental state is a certain role that is filled by an entity of some kind. Since materialism, stemming from the success of science, tells us that the occupants of such roles are neurological states, we can conclude that neurological states are mental states. In his first publication, ‘An Argument for the Identity Theory’ (Journal of Philosophy, 1966), Lewis outlines this formulation of the identity theory, which was independently articulated by D.M. Armstrong in A Materialist Theory of Mind (Routledge, 1968). A few years later, Lewis revisited the topic in ‘Mad Pain and Martian Pain’ (Readings in Philosophy of Psychology, 1980). Lewis had sent Armstrong a version of this paper in 1978, to which Armstrong replied with several comments. Lewis in reply to Armstrong’s comments wrote the following letter. It is our first letter of the month. (© Estate of David K. Lewis.)

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