Multiple Realizability - Fodor's Generalization

Fodor's Generalization

Jaegwon Kim took up the challenge of responding to the problems posed by multiple realizability for reductionist theories by suggesting that the physical realization base of a particular mental state was not a particular physical state but the disjunction of the physical states which realize it. Jerry Fodor replied to this objection by formulating a generalization of the multiple realizability thesis. According to Fodor, multiple realizability was not just something that occurred "across physical structure-types" but was a phenomenon that could occur even within the same token system (such as an organism). At different times, the same organism may realize type-identical mental kinds in physically different forms. (This thesis was later given some empirical support with the discovery of the relative plasticity of the human brain).

Fodor used this generalized multiple realizability thesis to argue against reductionism of the mind and of the special sciences. The key to Fodor's argument is that, in his characterization of reductionism, all mental kind predicates in an ideal and completed psychology must correspond with a physical kind predicate in an ideal and completed physics. He suggested taking Ernest Nagel's theory of reduction, which insisted on the derivability of all terms in the theory to be reduced from terms in the reducing theory and the bridging laws, as the canonical theory of reduction. Given generalized multiple realizability, the physical science part of these psychophysical bridge laws will end up being a, possibly infinite, disjunction of all the terms referring to possible physical realizations of a mental kind. This disjunction cannot be a kind-predicate and therefore the entire statement cannot be a law of physics. The special sciences cannot be reduced to physics in this way, according to Fodor.

Later on, in 1988, Hilary Putnam applied the argument from Fodor's generalized version of multiple realizability to argue against functionalism itself, including, and above all, his own version of functionalism, machine state functionalism. Noting that functionalism is essentially a watered-down reductionist or identity theory in which mental kinds are ultimately identified with functional kinds, Putnam argued that mental kinds were probably multiply realizable over functional kinds. The same mental state or property could be implemented or realized by different states of a universal Turing machine.