We ordinarily suppose that there is a difference between having and failing to exercise a rational capacity on the one hand, and lacking a rational capacity altogether on the other. This is crucial for our allocations of responsibility. Someone who has but fails to exercise a capacity is responsible for their failure to exercise their capacity, whereas someone who lacks a capacity altogether is not. However, as Gary Watson pointed out in his seminal essay ’Skepticism about Weakness of Will’, the idea of an unexercised capacity is much more difficult to make sense of than it initially appears. The aim of ’Rational Capacities’ is to provide the needed explication of this idea.