Mind: A Brief Introduction (John R. Searle) / quote 59 of 138
Mental states defined by their function
At this point the materialists made a move that was crucial for subsequent philosophizing about the mind. They said: what token brain states have that makes them mental states is a certain type of function in the overall behavior of the organism. Not surprisingly, this doctrine was called “functionalism,” and when spelled out it developed into views like the following:17 to say that Jones believes that it is raining is to say that he has a certain event, state, or process going on in him that is caused by certain sorts of external stimuli—for example, he perceives that it is raining; and this phenomenon, in conjunction with certain other factors, such as his desire to stay dry, will cause a certain sort of behavior on his part, the behavior of carrying an umbrella. Mental states, in short, are defined as states that have certain sorts of functions, and the notion of function is explained in terms of causal relations to external stimuli, to other mental states, and to external behavior.