A useful canonical visual representation of a mechanism underlying a phenomenon is shown in Figure 1 (from Craver 2007). At the top is the phenomenon, some system S engaged in behavior ψ. This is the behavior of the mechanism as a whole. Beneath it are the parts (the Xs) and their activities (the φs) organized together. The dotted roughly-vertical lines reflect the fact that the parts and activities are contained within, are components of, the mechanism engaged in this behavior. Thus represented, mechanisms are decompositional in the sense that the behavior of the system as a whole can be broken down into organized interactions among the activities of the parts.