The functionalists' causal theory of mind asserts that pain is a functional state: each type of mental state is a state consisting of a disposition to act in certain ways and to have certain mental states, given the appropriate sensory inputs and certain mental states.
According to Armstrong, " The present state of scientific knowledge makes it probable that we can give a purely physico-chemical account of man's body." He concludes that, "It will be natural to conclude that mental states are not simply determined by corresponding states of the brain, but that they are actually identical with these brain-states, brain states that involve nothing but physical properties."
In Lewis's version of functionalism, mental states are defined in a way that eliminates essential use of mental terminology but does not eliminate input and output terminology. Input and output terms correspond to actions and behaviors. Lewis claims that pain' is defined as a synonymous with a description containing input and output terms but no mental terminology.
Lewis explains, "Human pain is the state that occupies the role of pain for humans. Martian pain is the state that occupies the same role for Martians. The thing to say about Martian pain is that the Martian is in pain because he is in a state that occupies the causal role of pain for Martians, whereas we are in pain because we are in a state that occupies the role of pain for us." As for the madman, he is in pain, but he is not in a state that occupies the causal role of pain for him. He is in a state that occupies that role of a pattern of firing of neurons depends on one's circuit diagram, and he is hooked up wrong. Lewis concludes, "The thing to say about mad pain is that the madman is in pain because he is in the state that occupies the causal role of pain for the population comprising all mankind. The state that occupies the role for the population does not occupy it for him"
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