This article presents a distinct sense of ‘mechanism’, which I call the functional sense of
mechanism. According to this sense, mechanisms serve functions, and this fact places
substantive restrictions on the kinds of system activities ‘for which’ there can be a mechanism.
On this view, there are no mechanisms for pathology; pathologies result from disrupting
mechanisms for functions. Second, on this sense, natural selection is probably not
a mechanism for evolution because it does not serve a function. After distinguishing this
sense fromsimilar explications of ‘mechanism’, I argue that it is ubiquitous in biology and
has valuable epistemic benefits.