What is Functionalism in the Philosophy of Mind?

Functionalism in the philosophy of mind is a metaphysical view about mental states (e.g., beliefs, desires, perceptions, etc.). Qua metaphysical view, functionalism concerns what kinds of things mental states are, what their ultimate nature is. Functionalism arises as an alternative to the mind-brain identity theory. Like functionalism, the identity theory is a metaphysical theory: its aim is to say something about what the ultimate nature of mental states is. According to the identity theory, mental state types are identical with brain state types. Thus, for example, the theory will say that the mental state type pain is identical with a certain physico-chemical state type; e.g. C-fiber activation. One of the main intuitions behind functionalism is that a system could have mental states without having brains states like ours, or any brain states at all. A creature may be in pain regardless of whether it is in a brain state that is different from C-fiber activation, whether its “brain” is a hydraulic computer, etc. According to the functionalist, then, mental state types are not identical with brain state types. Instead, mental states are said to be realized or implemented by other (typically physical) states, be these physico-chemical states of brains, hydraulic states of hydraulic computers, etc. Thus the functionalist thinks that mental states are multiply realizable states. Different physico-chemical states, as well as hydraulic and electronic states, for example, can all realize or implement the same mental state type.

But what does functionalism take mental states to be, exactly? Functionalism dictates that mental states are (nothing but) functional states of organisms. To say that a mental state is a functional state is to say that there is nothing more to the state than its having a certain functional/causal role. The functional role of a mental state is the set of causal relations that the state bears to other mental states, sensory inputs, and behavioral outputs. To say that a mental state is a functional state, then, is to say that there is nothing more to the state than its being causally related in a certain way to sensory inputs, behavioral outputs, and other mental states. Thus, for example, a functionalist account of pain will say that pain just is a state that is typically caused by tissue damage, and that tends to cause winces and groans, etc.