During the 1960s, various philosophers such as Donald Davidson, Hilary Putnam, and Fodor tried to resolve the puzzle of developing a way to preserve the explanatory efficacy of mental causation and so-called "folk psychology" while adhering to a materialist vision of the world which did not violate the "generality of physics". Their proposal was, first of all, to reject the then-dominant theories in philosophy of mind: behaviorism and the type identity theory. The problem with logical behaviorism was that it failed to account for causation between mental states and such causation seems to be essential to psychological explanation, especially if one considers that behavior is not an effect of a single mental event/cause but is rather the effect of a chain of mental events/causes. The type-identity theory, on the other hand, failed to explain the fact that radically different physical systems can find themselves in the identical mental state. Besides being deeply anthropocentric (why should humans be the only thinking organisms in the universe?), the identity-type theory also failed to deal with accumulating evidence in the neurosciences that every single human brain is different from all the others. Hence, the impossibility of referring to common mental states in different physical systems manifests itself not only between different species but also between organisms of the same species.
One can solve these problems, according to Fodor, with functionalism, a hypothesis which was designed to overcome the failings of both dualism and reductionism. Without going into detail here, the idea is that what is important is the function of a mental state regardless of the physical substrate which implements it. The foundation for this view lies in the principle of the multiple realizability of the mental. Under this view, for example, I and a computer can both instantiate ("realize") the same functional state though we are made of completely different material stuff (see graphic at right). On this basis functionalism can be classified as a form of token materialism.
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