Putnam cited examples from the animal kingdom to illustrate his thesis. He asked whether it was likely that the brain structures of diverse types of animals realize pain, or other mental states, the same way. If they do not share the same brain structures, they cannot share the same mental states and properties, in which case mental states must be 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 other silicon-based life forms. These hypothetical entities, he contended, should not be considered incapable of experiencing pain just because they lack human neurochemistry. Putnam concluded that type-identity theorists had been making an "ambitious" and "highly implausible" conjecture that could be disproved by one example of multiple realizability. This is sometimes called the "likelihood argument".