Evolutionary biology
Putnam cites examples from the animal kingdom as evidence for the multiple realizability of mental states. Evolutionary biology — including evolutionary neuroscience — and comparative neuroanatomy and neurophysiology have demonstrated that mammals, reptiles, birds, amphibians, and mollusks have different brain structures. These animals can only share the same mental states and properties if these mental states and properties can be realized by different physical states in different species. Putnam concludes that type-identity and other reductive theories make an extremely "ambitious" and "highly implausible" conjecture that can be disproven with just one example of multiple realizability. On the contrary, it is likely that creatures that cannot be in identical physical states, due to their different composition and structure, can nevertheless be in identical psychological states. Some philosophers refer to this argument — that multiple realizability is much more likely than reductionism — as "the likelihood argument".