This paper is concerned with the semantics of Spanish ser capaz 'be capable' (henceforth SC). Specifically, it aims to account for the distribution SC displays and the range of meanings it expresses in the realm of abilities, viewed as kinds of dispositions. We put forth the following generalization: Whenever imperfective aspect interacts with SC, two possible readings arise, one that is purely abilitative and another one that expresses an accidental or unpredictable state of affairs. Our claim is that both interpretations derive from a circumstantial modal base that includes the subject's inner circumstances, albeit with a different ordering source that is brought about via the interaction of imperfective aspect and the syntax-semantics of the modal. Thus, we propose that the purely abilitative SC contains an ordering source that ranks as better worlds those in which the subject appeals to her strength of the body, intellect or character; the accidental reading is the result of covert movement of the adjective capaz 'capable' over the imperfective-bearing copula ser 'be', which triggers a change of ordering source in the modal.