The fact that mental terms seem to be applied in ensembles led a number of philosophers to think about technical ways of defining an entire set of terms together. Perhaps, they thought, words like belief, desire, thought, and intention could be defined in the way a physicist might simultaneously define mass, force, and energy in terms of each other and in relation to other terms. The American philosopher David Lewis (1941–2001) invoked a technique, called “ramsification” (named for the British philosopher Frank Ramsey [1903–30]), whereby a set of new terms could be defined by reference to their relations to each other and to other old terms already understood. Ramsification was based on an idea that had already been noted by the American philosopher Hilary Putnam with regard to the set of standard states of a computer. Each state in the set is defined in terms of what the machine does when it receives an input; specifically, the machine produces a certain output and passes into another of the states in the same set. The states can then be defined together in terms of the overall patterns produced in this way.

States of computers are not the only things that can be so defined; most any reasonably complex entity that has parts that function in specific ways will do as well. For example, a carburetor in an internal-combustion engine can be defined in terms of how it regulates the flow of gasoline and oxygen into the cylinders where the mixture is ignited, causing the piston to move. Such analogies between mental states and the functional parts of complex machines provided the inspiration for functionalist approaches to understanding mental states, which dominated discussions in the philosophy of mind from the 1960s.

Functionalism seemed an attractive approach for a number of reasons: (1) as just noted, it allows for the definition of many mental terms at once, avoiding the problems created by the piecemeal definitions of analytical behaviourism; (2) it frees reductionism from a chauvinistic commitment to the particular ways in which human minds happen to be embodied, allowing them to be “multiply realized” in any number of substances and bodies, including machines, extraterrestrials, and perhaps even angels and ghosts (in this way, functionalism is also compatible with the denial of type identities and the endorsement of token identities); and, most important, (3) it allows philosophers of mind to recognize a complex psychological level of explanation, one that may not be straightforwardly reducible to a physical level, without denying that every psychological embodiment is in fact physical. Functionalism thus vindicated the reasonable insistence that psychology not be replaced by physics while avoiding the postulation of any mysterious nonphysical entities as psychology’s subject matter.

However, as will emerge in the discussion that follows, these very attractions brought with them a number of risks. One worry was whether the apparent detachment of functional mental properties from physical properties would render mental properties explanatorily inert. In a number of influential articles, the American philosopher Jaegwon Kim argued for an “exclusion principle” according to which, if a functional property is in fact different from the physical properties that are causally sufficient to explain everything that happens, then it is superfluous, just as are the epiphenomenal angels that push around the planets. Whether something like the exclusion principle is correct would seem to depend upon exactly what relation functional properties bear to their various physical realizations. Although this relation is obviously a good deal more intimate than that between angels and gravitation, it is unclear how intimate the relation needs to be in order to ensure that functional properties play some useful explanatory role.

It is important to appreciate the many different ways in which a functionalist approach can be deployed, depending on the specific kind of functionalist account of the mind one thinks is constitutive of the meaning of mental terms. Some philosophers—e.g., Lewis and Jackson—think that the account is provided simply by common “folk” beliefs, or beliefs that almost everyone believes that everyone else believes (e.g., in the case of the mental, the beliefs that people scratch itches, that they assert what they think, and that they avoid pain). Others—e.g., Sidney Shoemaker—think that one should engage in philosophical analysis of possible cases (“analytical functionalism”); and still others—e.g., William Lycan and Georges Rey—look to empirical psychological theory (“psychofunctionalism”). Although most philosophers construe such functional talk realistically, as referring to actual states of the brain, some (e.g., Dennett) interpret it irreferentially—indeed, as merely an instrument for predicting people’s behaviour or as an “intentional stance” that one may (or equally may not) take toward humans, animals, or computers and about whose truth there is no genuine “fact of the matter.” In each case, definitions vary according to whether they are derived from an account of the whole system at once (“holistic” functionalism) or from an account of specific subparts of the system (“molecular” functionalism) and according to whether the terms to be defined must refer to observable behaviour or may refer also to specific features of human bodies and their environments (“short-armed” versus “long-armed” functionalism). Thus, there may be functional definitions of states of specific subsystems of the mind, such as those involved in sensory reception (hearing, vision, touch) or in capacities such as language, memory, problem solving, mathematics, and interpersonal empathy. The most influential form of functionalism is based on the analogy with computers, which, of course, were independently developed to solve problems that require intelligence. See also functionalism.