Multiple Realizability - Putnam's Formulation

Putnam's Formulation

A classic example of the thesis of multiple realizability is to be found in several papers published by Hilary Putnam in the late 1960s. In these papers, he argued that, contrary to the famous claim of type-identity theory, it was not true that "pain is identical to C-fibre firing." It is possible that pain corresponds to, or is at least correlated with, completely different physical states of the nervous system in different organisms and yet they all experience the same mental state of "being in pain." Putnam cited numerous examples from all over the animal kingdom to illustrate his thesis. Is it likely that the brain structures of all mammals, reptiles, birds, amphibians and molluscs realize pain, or other mental states, in exactly the same way? Do they even have the same brain structures? Clearly not, if we are to believe the evidence furnished by comparative neuroanatomy and neurophysiology. How is it possible then that they can share the same mental states and properties? The answer had to be that these mental kinds were realized by different physical states in different species. Putnam then took his argument a step further, asking about such things as the nervous systems of alien beings, artificially-intelligent robots and silicon-based life forms. Should such hypothetical entities be considered a priori incapable of experiencing pain just because they did not possess the same neurochemistry as humans? Putnam concluded that type-identity and other reductive theorists had been making an extremely "ambitious" and "highly implausible" conjecture which could be disproved with just one example of multiple realizability. This is sometimes referred to as the likelihood argument.

Putnam also formulated a complementary argument based on, what he called, functional isomorphism. He defined the concept in these terms: "Two systems are functionally isomorphic if there is a correspondence between the states of one and the states of the other that preserves functional relations." So, in the case of computers, two machines are functionally isomorphic if and only if the sequential relations among states in the first are exactly mirrored by the sequential relations among states in the other. Therefore, a computer made out of silicon chips and a computer made out of cogs and wheels can be functionally isomorphic but constitutionally diverse. Functional isomorphism implies multiple realizability. This is sometimes referred to as an "a priori argument".

Jerry Fodor, Putnam and others immediately noted that, along with being a very effective argument against type-identity theories, multiple realizability implied that any low-level explanation of higher-level mental phenomena would be insufficiently abstract and general. Functionalism, which attempts to identify mental kinds with functional kinds that are characterized exclusively in terms of causes and effects, abstracts from the physico-chemical level of microphysics and hence seemed to be a more suitable alternative explanation of the relation between mind and body. In fact, there are many functional kinds such as mousetraps, software and bookshelves which are multiply realized at the physical level.