In philosophy of mind, functionalism is a theory which, for methodological reasons, conceives of the mind as an information processing system and compares thought to a computation (Hobbes conceived the reason this way). Resulting from research in artificial intelligence, functionalism also adds a causal theory of mental states: mental states are linked together by the principle of causality.
To this extent, functionalism is not necessarily materialism: mental states can be hooked up to different media, be it the brain or the hardware and software of a computer, or any other medium. However, to the extent that the principle of causality is most often associated with matter, most functionalists are also materialists.
Functionalist theory has three types of specification:
- input specifications, specifications that state the kinds of things that cause mental states in people;
- internal state specifications that describe the causal interactions of mental states;
- output specifications that say what kinds of actions or behaviors are caused by mental states.
The Turing machine is an (abstract) example of a machine that can operate according to this model. A can dispenser works like this: it reacts to input (it is given 50 cents or one euro), and, depending on these, delivers the can or expects more cents.
According to John Searle, who in a way opposes Hilary Putnam’s interpretation that functionalism is compatible with a dualist thesis on mental and physical states, functionalism is on the contrary necessarily physicalism. None of these causes and effects should be conceived of as having a mental element. These are just physical sequences. The functionalist insists that it be understood that he is not saying that a belief is an irreducible mental state which, in addition to the causal relations which are its own, but rather that a belief consists only in that it has these causal relationships. A belief can be a bundle of neural stimuli, or the electronic tension level of a computer, or even Martian green slime, or whatever, as long as it’s part of the right pattern relationships of cause and effect. A belief, therefore, is only one thing, an X, that is part of the scheme of causal relations, and it is such because it lies in such and such a place in the scheme of causal relations.
To this extent, functionalism is akin to methodological behaviorism: unlike ontological behaviorism, it does not assert that there are no mental states. But, like methodological behaviorism, it leaves aside the subjective, qualitative aspects of mental states (or qualia), that is to say the set of subjective experiences that can be the subject, for example, of a poem or a declaration of love, or of the simple fact of loving a certain color. We thus speak of “black box functionalism”.