 Aristotle refers to potencies in criticizing some of his contemporaries who claimed that only that which takes place is possible (Met. IX.3). The model of possibility as potency prima facie allowed him to speak about all kinds of unrealized singular possibilities by referring to passive or active potencies, but taken separately they represent partial possibilities which do not guarantee that their actualization can take place. More is required for a real singular possibility, but when the further requirements are added, such as a contact between the active and passive factor and the absence of an external hindrance, the potency model suggests that the potency can really be actualized only when it is actualized (Met. IX.5, Phys. VIII.1). It is possible that this led Aristotle to define motion (kinêsis) as the actuality of the potentiality (of the end) qua potentiality (Phys. III.1), but this did not explain the possibility of beginning (Hintikka et al. 1977).