Mechanisms, Multiple Realizability, and Medium Independence

Abstract

Abstract This chapter provides an account of realization within a mechanistic framework and introduces the notions of variable realizability, multiple realizability, and medium independence. Realization is the relation between a higher-level property and the lower-level properties of which it is an aspect. Variable realizability occurs when the same higher-level property can be realized by different lower-level properties—different lower-level properties share the same aspect. Variable realizability is ubiquitous yet insufficient for multiple realizability proper. Multiple realizability proper occurs when the same higher-level property can be realized by different lower-level properties that constitute different mechanisms for that property at the immediately lower mechanistic level. Medium independence is an even stronger condition than multiple realizability: it occurs when not only is a higher-level property multiply realizable; in addition, the inputs and outputs that define the higher-level property are also multiply realizable. Thus, all that matters to defining a medium-independent higher-level property is the manipulation of certain degrees of freedom. Medium independence entails multiple realizability, which in turn entails variable realizability, but variable realizability does not entail multiple realizability, which in turn does not entail medium independence.