  Non-reductive physicalism in its current form grew out of functionalism, according to which mental properties are functional properties. To be in pain, for example, is a matter of being in a state with a certain causal profile, a state that’s caused by tissue damage, and causes certain overt responses (moans, attempts to repair the damage, beliefs that one is in pain). But, argue functionalists, it is most unlikely that we could identify a single kind of physical state playing this role in every actual and possible case of pain. Human beings differ in endless tiny physiological ways: your neurological states, including states you go into when you are in pain, probably differ subtly from another person’s. Human beings’ neurological states, in turn, differ from those of a cat or a dog, and perhaps dramatically from states of an octopus. You might even imagine encountering aliens with vastly different biologies, but to which you would unhesitatingly ascribe pains.