A theory in Philosophy of Mind, which maintains that mental states are best understood in relation to 1) their sensory stimulation or input (cause(s)), 2) other mental states, and 3) their behavioral effects. Functionalism implies that insofar as something instantiates such a system, it can be said to possess mental states, regardless of the physical properties peculiar to the matter that composes it, e.g., a silicon-based object representing such a system could be said to have mental states.

Functionalism differs from behaviorism and some forms of identity theory in Philosophy of Mind in that the former considers only stimuli and behavioral effects in attributing mental states to objects (under this interpretation a urinal or microwave could be seen as having mental states) and in many of the latter, the physical properties of an object either permit or preclude it from possessing mental states (the human brain would be an example of an object whose physical properties would, under some forms of identity theory, allow it to possess mental states, while Martian brains would not, since they (mental states in general) would be identical to human brain states, thus preventing Martians from being sentient).

While functionalism is apparently more inclusive with respect to the types of things that may be considered to possess mental states while also not attributing them to objects one would find it absurd to think were conscious, there is strong criticism of this view. One of the most prominent objections concerns the qualitative aspects (or qualia) of mental states, e.g. "what it is like" to taste chocolate or see the colour red, which, it is held, are essential to mental states and are entirely ignored by funcationalist accounts thereof. One well known example of this objection is the "inverted spectrum" thought experiment, which consists of two persons A and B perceiving qualitatively different colours when both viewing light reflected off an object at a certain wavelength, e.g. one sees red as red and the other red as blue. While both persons A and B have the same sensory stimuli and their mental states stand in the same causal relation to their other mental states and behavioral output (both would assent if asked if the colour they saw was red), they have qualitatively different experiences.

The most famous articulations of this can be found in Thomas Nagel's "What is it like to be a Bat", Philosophical Review, LXXXIII, 4 (October 1974) pp. 435 - 50, and Frank Jackson's essays "Epiphenomenal Qualia", Philosophical Quarterly, 32 (1982) pp. 127 - 36, and "What Mary Didn't Know", The Journal of Philosophy, LXXXIII, 5 (May 1986) pp. 291 -95