 Thus far our questions about abilities have been formal ones: we have been asking what it is to have an ability without concerning ourselves with the substantive work that a theory of ability might have to do. But there is much work to be had for a theory of ability: abilities have figured as unexplained explainers in a range of philosophical theories, for example in accounts of concepts (Millikan 2000), of knowledge (Greco 2009, Sosa 2007), and of ‘knowing what it’s like’ (Lewis 1988). Perhaps the most prominent substantive role for a theory of ability has been the uses to which accounts of ability have been put in the free will debates. So let us close with a brief survey of what work a theory of ability might be expected to do in those debates.