 In De Caelo I.12 Aristotle supposes, per impossibile, that a thing has contrary potencies, one of which is always actualized. He argues that the alleged eternally unactualized potencies are not potencies at all because they cannot be assumed to be realized at any time without contradiction. Aristotle applies here the model of possibility as non-contradictoriness which is defined in Prior Analytics I.13 as follows: when a possibility is assumed to be realized, it results in nothing impossible. In speaking about the assumed non-contradictory actualization of a possibility, Aristotle thinks that it is realized in real history. Aristotle uses in De Caelo I.12 and in some other places (for example, Met IX.4, An. pr. I.15) a reductio argument which consists of assuming the denial of what is to be proved and showing that this leads to a contradiction. The argument has created much controversy about how possibilities are supposed to obtain. See Judson 1983; Rini 2011, 135–156; Rosen and Malink 2012; Smith 2016.) 