The phenomenon is the behavior of the mechanism as a whole. All mechanisms are mechanisms of some phenomenon (Kauffman 1971; Glennan 1996, 2002). The mechanism of protein synthesis synthesizes proteins. The mechanism of the action potential generates action potentials. The boundaries of a mechanism—what is in the mechanism and what is not—are fixed by reference to the phenomenon that the mechanism explains. The components in a mechanism are components in virtue of being relevant to the phenomenon.