Some philosophers accept the thesis that mental states are multiply realizable but deny that multiple realizability gives rise to functionalism or other forms of non-reductive physicalism.
Reductionism in other sciences Early objections to multiple realizability were limited to the narrow, "across structures-type" version. Starting with
David Kellogg Lewis, many reductionists argued that it is very common in scientific practice to reduce one theory to another via local, structure-specific reductions. A frequently cited example of this sort of
intertheoretic reduction is
temperature. The temperature of a gas is identical to mean molecular kinetic energy. Temperature in a solid is identical to mean
maximal molecular kinetic energy because the molecules of a solid are more restricted in their movements. Temperature in a
plasma is a mystery because the molecules of a plasma are torn apart. Therefore, temperature is multiply realized in a diversity of microphysical states.
Disjunction Jaegwon Kim has argued that disjunction—the idea that the physical realization of a particular mental state is not a particular physical state but the disjunction of the physical states that realize that mental state—creates problems for multiple realizability. Putnam also argued against this "disjunctive" possibility in earlier work.
Causal closure of the physical Jaegwon Kim has argued against non-reductive physicalism on the grounds that it violates the
causal closure of the physical, which assumes that physics provides a full explanation of physical events. If mental properties are causally efficacious, they must either be identical to physical properties or there must be widespread
overdetermination. The latter is often held to be either unlikely or even impossible on conceptual grounds. If Kim is right, then the options seem to be either reduction or elimination.
Insufficient generalizability One criticism of multiple realizability is that any theory that attempts to address the possibility of
generalized multiple realizability must necessarily be so local and context-specific in nature (referring exclusively to a certain token system of a certain structure-type at a certain time) that its reductions would be incompatible with even a minimally acceptable degree of generality in scientific theorizing. Any psychology that is sufficiently narrow to accommodate this level of multiple realizability required to account for neural plasticity will almost certainly not be general enough to capture the generalizations needed to explain human psychology. Some reductionists reply that this is not empirically plausible. Research and experimentation in the neurosciences requires that some universal consistencies in brain structures must either exist or be assumed to exist. The similarity (produced by
homology or convergent evolution) of brain structures allows us to generalize across species. If multiple realizability (especially the generalized form) were an empirical fact, then results from experiments conducted on one species of animal (or one organism) would not be meaningful or useful when generalized to explain the behavior or characteristics of another species (or organism of the same species; or in the generalized form, even the same organism).
Sungsu Kim has recently responded to this objection using the distinction between homology of brain structures and
homoplasy. Homologies are characteristics of physiology, morphology, behavior, or psychology shared by two or more species and inherited from a common ancestor. Homoplasies are similar or identical characteristics shared by two or more species but not inherited from a common ancestor, having evolved independently. The feet of ducks and platypuses are an example of homoplasy, while the hands of humans and chimps are an example of homology. The fact that brain structures are homologous provides no evidence either for or against multiple realizability. The only way to empirically test the thesis of multiple realizability would be to examine brain structures and determine whether some homoplasious "psychological processes or functions might be 'constructed' from different material" and supported by different brain structures just as the flight capacities of bats and birds emerge from different morphophysiologies. The emergence of similar behavioral outputs or psychological functions brought about by similar or identical brain structures in
convergent evolutionary lineages would provide some evidence against multiple realizability, since it is highly improbable that this would happen, if not for constraints on the type of physical system that can realize mental phenomena. This, however, would not completely refute the possibility of realizability of mental states in radically different physical systems such as non-
carbon based life forms or machines. == Historical note ==