 In contrast, for the functionalist (Fodor 1997) mental kinds are defined by their functional roles, rather than by the essential properties of the neurophysiological kinds that realize them. It does not matter which physical kind does the realizing, be it silicon chips, neurons, C-fibres or whatever. Rather, what matters is that the psychological state (e.g., pain) plays a certain functional role. The whole point of multiple realizability is that it is easy to imagine some possible world where a token mental kind is realized by silicon chips, C-fibres firing, or whatever. So, a mental kind is not essentially realized by a certain neurophysiological kind; rather it plays the same functional role independently of what underlying kind is realizing it. This appears to violate Criterion 1 for natural kinds (in Section 1.1.1), that members of a natural kind should have some natural properties in common.