 The multiple realizability contention about the mental is that a given psychological kind (like pain) can be realized by many distinct physical kinds: by different brain states in earthly biological pain-bearers, by electronic states in properly programmed digital computers, by green slime states in imagined extraterrestrials, and so on. Correctly characterizing the realization relation remains a contentious matter in analytic metaphysics (Gillett 2003; Polger 2004) and this issue quickly reaches detailed technical depths. But whatever the correct account of realization turns out to be, about whatever kinds turn out to be related by realization, the multiple realizability contention about the mental holds that a given psychological kind (like pain) can stand in that relationship to many distinct physical kinds. Further discussion of this issue with numerous references will arise in  section 3  below.