 Another Aristotelian modal paradigm was that of possibility as a potency. In Met. V.12 and IX.1 potency is said to be the principle of motion or change either as the activator or as the receptor of a relevant influence. (For agent and patient in Aristotle’s natural philosophy in general, see Waterlow 1982b.) The types of potency-based possibilities belonging to a species are recognized as possibilities because of their actualization — no natural potency type remains eternally frustrated. Aristotle says that when the agent and the patient come together and there is no hindrance, the one must act and the other must be acted on (Met. IX.5).