 One quick word on this distinction between multiple realizability and realization, for it marks a point where treatment of this topic in metaphysics and cognitive science diverged. Metaphysicians, many of whom came quickly to reject the contingent identity claims of the early mind-brain identity theorists in favor of the necessity of identity claims, focused on multiple realizability, since the possibility of distinct physical realizers of the same psychological kind was sufficient to block any such identity between realized kind and any one of its possible realizers. Philosophers who took their cue from the cognitive and brain sciences focused on proposed instances of multiple realization, of actual instances of the relation among existing cognizers. The emphasis in this entry will be on the latter issue, although some discussion of the metaphysical issues will arise in  section 2  and  section 3  below. 