has also 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