 Suppose that, in humans, there is some distinctive kind of neural activity (C-fiber stimulation, for example) that meets these conditions. If so, then according to this functionalist theory, humans can be in pain simply by undergoing C-fiber stimulation. But the theory permits creatures with very different physical constitutions to have mental states as well: if there are silicon-based states of hypothetical Martians or inorganic states of hypothetical androids that also meet these conditions, then these creatures, too, can be in pain. As functionalists often put it, pain can be realized by different types of physical states in different kinds of creatures, or multiply realized. (See entry on  multiple realizability.)  Indeed, since descriptions that make explicit reference only to a state's causal relations with stimulations, behavior, and one another are what have come to be known as “topic-neutral” (Smart 1959) — that is, as imposing no logical restrictions on the nature of the items that satisfy the descriptions — then it's also logically possible for non-physical states to play the relevant roles, and thus realize mental states, in some systems as well. So functionalism is compatible with the sort of dualism that takes mental states to cause, and be caused by, physical states.