 In addition, Putnam (1967) points out that early mind-brain identity theorists insisted that these identities, while contingent, hold by virtue of natural (scientific) law. So then any physically possible pain-bearer must also be capable of possessing that physical-chemical kind. This point marks the introduction of the crucial distinction between multiple realization and multiple realizability into the discussion, as well as the well-known philosophers’ imagined metaphysical possibilities. Silicon-based androids, artificially intelligent electronic robots, and Martians with green slime pulsating within their bodies all seem to be possible pain realizers. But they lack “brain states” comparable to ours at any level of physical description. Further still, these mind-brain identity theories were supposed to be completely general. Every mental kind was held to be identical to some neural kind. So the critic needs to find only one mental kind, shared across these structure-types yet realized differently at the physical-chemical level. Putnam (1967) acknowledges that the early identity theories were being offered an empirical hypothesis. But this consequence was “certainly ambitious”, and almost certainly false.