 So, as the term will be understood here, an ability is a power that relates an agent to an action. This way of demarcating abilities, while serviceable for our purposes, is not unproblematic. For it inherits the problems involved in drawing the distinction between actions and non-actions. First, there is the problem that the domain of action is itself a contentious matter. Second, there is the problem that, even if we have settled on an account of action, it is plausible that the domain of action will be vague, so that there are some events that are not definitely actions, but that are not definitely not actions either. Arguably both of these points about action apply, also, to the property of being an agent. If this is right, then the present account of ability, which is cashed out in terms of agency and action, will be correspondingly contentious and vague. Borderline cases may, in the end, generate problems for the theory of ability. But such problems will not be central here. For giving such a theory will be difficult enough even when we focus on paradigm cases of agency and action, and so on paradigm cases of ability.