Functionalism is the dominant theory of mental states in modern philosophy. Functionalism was 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 medium (the neurons), instead attending to higher-level functions such as beliefs, desires, and emotions.

According to functionalism, the mental states that make up consciousness can essentially be defined as complex interactions between different functional processes. Because these processes are not limited to a particular physical state or physical medium, they can be realized in multiple ways, including, theoretically, within non-biological systems.

This affords consciousness the opportunity to exist in non-human minds. According to some materialist views of consciousness, only a biological human brain was capable of consciousness. Everything that consciousness was, the argument went, was surely unique to and could be traced to a particular center of the biological human brain. However, it has been shown that that the human brain has "functional plasticity" such that people with as much as half of their brains removed during early infancy apparently can develop into adults whose behavior cannot be distinguished from other adults with all of their original brain intact. According to the materialist view of the mind, these people could not have consciousness because their radically different biological mind could not have all of the same physical features of a normal mind. Functionalism accommodates this problem easily by holding that the exact biological structures of the mind need not be the same, just so long as the same process was achieved. This could allow a computer or artificial intelligence to have consciousness, if we accept that cognitive processes are computational.

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" capable of computing any given algorithm (i.e. a Turing machine). This machine would involve:

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

So long as the same process was achieved, the "physical stuff" -- that being computer hardware or biological structure -- could achieve consciousness.

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
Putnam is also responsible for the Twin Earth thought experiment which was intended as an argument against semantic internalism; that is, meanings deriving, at least partially, from inside the mind. Some take the experiment as a criticism of functionalist theory's ability to meaningfully characterize mental states.

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.