Why Everything Doesn't Realize Every Computation

Some have suggested that there is no fact to the matter as to whether or not a particular physical system realizes a particular computational description. This suggestion has been taken to imply that computational states are not "real", and cannot, for example, provide a foundation for the cognitive sciences. In particular, Putnam has argued that every ordinary open physical system realizes every abstract finite automaton, implying that the fact that a particular computational characterization applies to a physical system does not tell one anything about the nature of that system. Putnam's argument is scrutinized, and found inadequate because, among other things, it employs a notion of causation that is too weak. I argue that if one's view of computation involves embeddedness (inputs and outputs) and full causality, one can avoid the universal realizability results. Therefore, the fact that a particular system realizes a particular automaton is not a vacuous one, and is often explanatory. Furthermore, I claim that computation would not necessarily be an explanatorily vacuous notion even if it were universally realizable. Key words. Computation, philosophy of computation, embeddedness, foundations of cognitive science, formality, multiple realization.