Functionalism is a theory in philosophy developed as an answer to the mind-body problem because of objections to both identity theory and logical behaviourism. Its core idea is that the mental states can be accounted for without taking into account the underlying physical substrate (the neurons), instead attending to higher-level functions such as beliefs, desires, and emotions.

Functionalism's explanation of consciousness, or the mental, is best understood when considering the analogy made by functionalists between the mind and the modern digital computer. More specifically, the analogy is made to a "machine" that the Church-Turing thesis posits as capable in principle of computing any given algorithm (i.e. as having the capabilites of a Turing machine). This machine would involve:

1. Data input (the senses in humans).
2. Data output (both behaviour and memory).
3. Functional states (mental states).
4. The ability to move from one functional state into another.
5. The definition of functional states with reference to the part they play in the operation of the entire entity - ie. in reference to the other functional states.

This variety of functionalism was developed by Hilary Putnam. One of the major proponents of functionalism is Jerry Fodor.

Criticism: Putnam's Twin Earth thought experiment

Curiously, Putnam is also responsible for the Twin Earth thought experiment which was initially intended as an argument against the internalism of word meanings; that is, meanings deriving, at least partially, from inside the mind. Some take the experiment as decisively defeating functionalism. The experiment is simple and runs as follows. Imagine a Twin Earth which is identical to Earth in every way but one: water is not H₂0, it's a substance XYZ. It is absolutely critical, however, to note that XYZ on Twin Earth is still called 'H₂0' even though it is a different substance (i.e. the one we call 'XYZ' on Earth). Since these worlds are identical in every way but one, you and your Twin Earth Doppelganger see exactly the same things, meet exactly the same people, have exactly the same jobs, and behave exactly the same way. In other words, you share the same inputs, outputs, and relations between inputs and outputs. But there's one crucial difference. You know (or at least believe, if we wish to make a weaker claim or avoid epistemological issues) that water is H₂0. Your Doppelganger knows that water is XYZ. Therefore, you differ in mental states though the causal properties that define your mental states are identical.

Various other counter-arguments to functionalism have been offered such as the modal one by Kripke in Identity and Necessity.

See also

• Consciousness
• philosophy of mind
• cognitive science
• simulated consciousness