The near decomposability of mechanisms is directly related to the idea that mechanisms span multiple levels of organization. The behavior of the whole is explained in terms of the activities and interactions among the component parts. These activities and interactions are themselves sustained by underlying activities and interactions among component parts, and so on (see Bechtel and Richardson 2010 [1993]).  Craver (2007) defines levels of mechanisms in terms of a relationship between the behavior (ψ) exhibited by a system (S) and the activity (φ) of some component part (X) of that system. On this account, X’s φ-ing is at a lower level of mechanistic organization than S’s ψ-ing if and only if (i) X is a part of S, and (ii) X’s φ-ing is a component in S’s ψ-ing. In short, to say that something is at a lower mechanistic level than the mechanism as a whole is to say that it is a working part of the mechanism. Though the term “level” is used in many legitimate ways, levels of mechanisms seem to play a central role in structuring the relations among many different models in contemporary biology (e.g., between Mendelian and molecular genetics (Darden 2006), between learning and memory and channel physiology (Craver 2007), and between population-level variation and developmental mechanisms (Tabery 2009, 2014a)).