 Early medieval thinkers were well acquainted with ancient modal conceptions through Boethius’s works. One of the Aristotelian modal paradigms occurring in Boethius is that of possibility as potency (potestas, potentia). According to Boethius, when the term ‘possibility’ (possibilitas) is used in the sense of ‘potency’, it refers to real powers or tendencies, the ends of which are either actual or non-actual at the moment of utterance. Some potencies are never unrealized. They are said to be necessarily actual. When potencies are not actualized, their ends are said to exist potentially (In Periherm. II, 453–455). Necessarily actual potencies leave no room for the potencies of their contraries, for they would remain unrealized forever and the constitution of nature does not include elements which would be in vain (In Periherm. II, 236). The potencies of non-necessary features of being do not exclude contrary potencies. They are not always and universally actualized, but as potency-types even these potencies are taken to satisfy the actualization criterion of genuineness (In Periherm. I, 120–121; II, 237).