Power is often taken to be a central concept in social and political thought that can contribute to the explanation of many different social phenomena. This article argues that in order to play this role, a general theory of power is required to identify a stable causal capacity, one that does not depend on idiosyncratic social conditions and can thus exert its characteristic influence in a wide range of cases. It considers three promising strategies for such a theory, which ground power in (1) the ability to use force, (2) access to resources, or (3) collective acceptance. It shows that these strategies fail to identify a stable causal capacity. The lack of an adequate general theory of power suggests that the concept lacks the necessary unity to play the broad explanatory role it is often accorded.