 One influential account of metaphysical structure, especially in the philosophy of mind, has been functionalism. Here is a simplified toy example of a functional analysis: any system that takes some money as an input, and reliably produces a candy as an output, thereby counts as a candy machine. On this account, the kind candy machine is individuated by input-output relations. A functional kind is any kind that can be individuated in this way. Because functional kinds are not individuated by the nature of the stuff that realizes the functional relations, they are often claimed to be paradigmatically friendly to multiple realization. Thus, given my characterization of candy machines, such a machine could be realized by a structure composed of metal or of plastic or perhaps even of spooky supernatural stuff. In light of this possibility of multiple realization, the relationship of functionalism to reduction is controversial: many philosophers have taken multiple realizability to constitute a barrier to reduction, but others disagree. (See the entries on  functionalism  and  multiple realization  for useful discussion).