 The bulk of theories of ability that have been defended in the historical and contemporary literature have been what we might call hypothetical theories. On such views, to have an ability is for it to be the case that one would act in certain ways if one were to have certain volitions. One arrives at different theories depending on how one understands the volitions in question and how exactly these actions would hypothetically depend on them, but nonetheless these views constitute something like a unified family. Given their prominence and unity, it is natural to begin our survey of theories of ability with them.