A functionalist theory also might characterize as pain that tends to be caused by bodily injury, to let us believe that there is something is wrong with our body and because of this we tend to have to anxiety, and, in the absence of any stronger, conflicting desires, to cause crying 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. With this in mind there are three senses of functionalism: (a) decompositional functionalim, (b) computation-representation functionalism, and (c) metaphysical functionalism. Decompositional functionalim is a study that relies on a system of decomposition into its components; the whole system is then explained in terms of these functional parts. Computation-representation functionalism relies heavily on the 'computer-as-mind' analogy. Metaphysical functionalism "is a theory of mind that hypothesizes that mental states simply are functional states. The metaphysical functionalist claims that mental states are the types of mental state they are because of the causal relations between inputs, outputs and other mental (i.e. functional) states of the system" (http://artsci.wustl.edu/philos/MindDict/F.html). In general, the three senses of functionalism make the accusation that the physical realization of a given function is not, in some sense, its essence. Metaphysical functionalism identifies causal structures with mental states which are realizable by "a vast variety of physical systems" (Block 1980, p. 173). Metaphysical functionalism often identifies mental states with Turing machine "table states" (Block 1980, p. 172). Like metaphysical functionalism, computation-representation functionalism holds the information in the brain. Lastly, decompositional functionalism is, superficially at least, more interested in the function of a system than the physical makeup of the system. However, decompositional functionalism is very seldom used. While it is true that in decompositional functionalism the function of the system being explored is often abstracted from its physical realization, it is also being continuously re-examined in order to better understand and characterize its function. As a theory of mind, functionalism is quite appealing. It is based in mathematical proof and provides a means of constructing analogies to guide our understanding of the mind. Computation-representation functionalism is a clear example of such a 'rigorous' analogy. In fact, because both digital computers and people are presumably Turing machine describable, the computer/brain analogy can be supported by direct reference to computational theory. This is how Turing machine equivalence has played such a central role in supporting functionalist intuitions (Fodor 1981).
Dualism is "the view that reality consists of two disparate parts. In philosophy of mind, the belief that the mental and physical are deeply different in kind: thus the mental is at least not identical with the physical" (http://www.artsci.wustl.edu/philos/MindDict/dualism.html.) Identity Theory, unlike dualism and functionalism explains the interaction between the mind and body as the physical processes in the