Mental state M == Functional state F
F is characterised by its causal relations - Ms are causally related to each-other and to input and outputs.
Sensory input leads to sequential mental states M1, M2, M3 ..., each of which may lead to behavioural outputs B1, B2, B3 ...
* The Ms are causally related.
* With the additional premise that all these causes and effects are physical in nature, functionalism becomes a materialistic theory.
The same Ms may be realised in different physical hardwares.
Functionalism aims to provide a philosophical framework for a science of psychology.
Functionalism addresses the classic mind-body problem: thought can occur in a physical apparatus, it can be correlated with a certain behaviour, yet we can talk about it without constantly identifying the precise activities happening in the physical apparatus. We can talk of mental events and of one event causing another, without any need to reduce psychological explanations to the neuro-physiological level. According to functionalism mentalistic concepts may never become redundant.
Fodor's illustration of two coke machines, one behaviouristic and one mentalistic (Scientific American January 1981, p.131) shows how the first is defined by input and output only while the second needs to be described in terms of interdependent ``mental'' states.
But does functionalism merely encourage tautological ``explanations''? Functional explanations and interdependent mental states typify this approach but they cannot be sufficient. Thus the added constraint: a plausible mechanism should exist to carry out the postulated functions, or, mental processes should be computable by a Turing machine. The mind is a symbol-processing device.
Turing machine functionalism: All individuals are realisations of the Universal Turing machine. (i.e. their program can be reduceable to the UTM program)
(Variants - Machine functionalism, Psychofunctionalism ... we will continue the discussion with generic functionalism)
Since functionalism does not subscribe to any particular version of the mapping from psychological to physiological, it is a conservative position, offering no bold conjecture to test.