According to the doctrine of functionalism within the philosophy of mind, a mental state "does not depend on its internal constitution, but rather on the way it functions, or the role it plays, in the system of which it is a part"(Levin, 2013).
Stemming ultimately from Aristotle's understanding of the human soul, it was only relatively recently, in the latter part of the 20th century, that functionalism, in the philosophy of mind, became a highly articulated position within the philosophy of mind(Levin, 2013). For the functionalist, the notion of the mind as a holistic system is essential. "More precisely, functionalist theories take the identity of a mental state to be determined by its causal relations to sensory stimulations, other mental states, and behavior"(Levin, 2013).
"For (an avowedly simplistic) example, a functionalist theory might characterize pain as a state that tends to be caused by bodily injury, to produce the belief that something is wrong with the body and the desire to be out of that state, to produce anxiety, and, in the absence of any stronger, conflicting desires, to cause wincing or moaning. According to this theory, all and only creatures with internal states that meet these conditions, or play these roles, are capable of being in pain"(Levin, 2013).
For Aristotle, entities are articulated in terms of their functions, with their functions being determined according to their "form." It is the form of an entity, that is, which allows it to fulfill its intended function. The form/function of the eye is to see, the form/function of the ear is to hear, and the form/function of the soul is the human body, which gives the human his vital capacities.
"the soul is to be identified with whatever powers and capacities enable a natural, organized human body to fulfill its defining function, which, according to Aristotle, is to survive and flourish as a living, acting, perceiving, and reasoning being. So, Aristotle argues, the soul is inseparab le from the body, and comprises whichever capacities are required for a body to live, perceive, reason, and act"(Levin, 2013).
Functionalism has interesting consequences for our understanding of the relation of physical brain-states to mental states. It is conducive to the doctrine of multiple realizability, for example, according to which it is possible for creatures with very differently constituted nervous systems to experience roughly equivalent states. This is because all that is required to count as a mind is to have a mental state that serves a specific function in a system. Any sort of physiologically constituted nervous system, therefore, could hypothetically serve as the ground for a system of mental states, provided its mental states meet the functionalist criteria of what a mind is.
"Indeed, since descriptions that make explicit reference only to a state's causal relations with stimulations, behavior, and one another are what have come to be known as “topic-neutral” (Smart 1959) — that is, as imposing no logical restrictions on the nature of the items that satisfy the descriptions — then it's also logically possible for non-physical states to play the relevant roles, and thus realize mental states, in some systems as well. So functionalism is compatible with the sort of dualism that takes mental states to cause, and be caused by, physical state"(Levin, 2013).
Although it paves the way for dualism, it is no less tantalizing an alternative for certain materialists who reject the Psycho-Physical Identity Thesis (Levin, 2013). Such philosophers reject the notion that animals with brains unlike ours are incapable of experiencing comparable sensations or feelings.
The more 'computational' approach to a functionalist philosophy of mind was anticipated by Thomas Hobbes, who saw thinking purely in terms of calculation according to a set of fixed and mechanized principles. Indeed, he describes the workings of the mind as a kind of automated machine which has its own wheels, engines, springs, etc. (Levin, 2013). Understanding the human mind as a computational system in such a way became particularly popular among functionalists of the 20th century.
Functionalism had one of its watershed moments when A.M. Turing contemplated the possibility that a machine with a sufficiently sophisticated set of finite instructions could fool a human into thinking that it is another human. The implications of such a machine for the doctrine of multiple realizability are obvious. Indeed, Turing himself believed that such a thing was theoretically possible. He
"identifies thoughts with states of a system defined solely by their roles in producing further internal states and verbal outputs, a view that has much in commonw tih contemporary functionalist theories. Indeed, Turing's work was explicitly invoked by many theorists during the beginning stages of 20th century functionalism, and was the avowed inspiration for a class of theories, the "machine state" theories most firmly associated with Hilary Putnam"(Levin, 2013).
A comparison of functionalism with certain elements of behaviorism is instructive. According to behavioristic psychology, only behavioral dispositions ought to be taken seriously as existing, since they alone, rather than thoughts, feelings, or other psychological states, can be directly observed.
The motivation for this had to do with the desire to cause psychology to be a maximally scientific discipline. Furthermore, behaviorists were concerned about the notion that psychological representations are generated by an internal "homunculus," whose nature would itself require explanation; something which was not in accord with the sort of rigor psychology as a science would demand.
Indeed, the behaviorists wanted the science of psychology to be as objective as the harder sciences like chemistry or physics (Levin, 2013). Behaviorism met its well-known end, however, at the hands of Chomsky, who argued that behaviorists tacitly relied upon mental states.
For example, while hunger may have been operationalized in terms of time between feeding or food-acquiring activity, it presupposed knowledge of the internal, subjective experience of hunger in rats. Likewise, humans who were involved in behavioristic experiments wanted to cooperate with the experimenters, presupposing a complex of psychological motivations and the ability to understand complex sets of instructions (Levin, 2013).
Thus, psychology could be plenty objective even while taking seriously the reality of internal psychological states, as long as it did not rely solely upon introspection, as Levin (2013) notes, and as long as these states demonstrably played a relevant role in the behavior being examined.
A distinction is to be made between this more popular behaviorism, and logical behaviorism. According to logical behaviorism, when we speak of mental states, we are speaking about behavioral tendencies. So for example, to say that "Felipe likes chow chows" might entail that "Felipe is predisposed to petting chow chows and squeezing their cheeks."
Like the aforementioned behaviorism, this behaviorism has no reference whatsoever to internal psychological states. This is not quite plausible, unfortunately, since Felipe may like chow chows but not be inclined to pet chow chows or smush their faces because he knows that chow chows are some of the most aggressive breeds there are, and he is afraid of getting bitten. Levin (2013) likewise uses the example of an actor exhibiting a pain-detecting behavior even if he is not really in pain. In sum:
"The problem, these philosophers argued, is that no mental state, by itself, can be plausibly assumed to give rise to any particular behavior unless one also assumes that the subject possesses additional mental states of various types. And so, it seemed, it was not in fact possible to give meaning-preserving translations of statements invoking pains, beliefs, and desires in purely behavioristic terms. Nonetheless, the idea that our common sense concepts of mental states reveal an essential tie between mental states and their typical behavioral expressions is retained, and elaborated, in contemporary “analytic” functionalist theories"(Levin, 2013).
Levin, Janet, "Functionalism", The Stanford Encyclopedia of Philosophy (Fall 2013 Edition), Edward N. Zalta (ed.), URL = <plato.stanford.edu/archives/fall2013/entries/functionalism/>.