In De Caelo A. 12. 281a28-b25 Aristotle presents an argument in support of the claim that ‘everything that always is, is absolutely indestructible’. Scholars are in agreement about at least one thing: this argument is notoriously difficult to understand. This paper attempts to do a number of things. First, it critically evaluates Lindsay Judson’s interpretation of De Caelo 281a28-b25, and it then proceeds to offer an alternative reconstruction of this passage. Second, it shows that Aristotle’s argument in this stretch of text is indeed problematic, but that the problem with it is not the one identified by Judson. On the basis of this reading of De Caelo 281a28-b25, the paper goes on to make the following points: (a) the argument from De Caelo A. 12, in conjunction with material from elsewhere in the corpus, may be used to show that Aristotle is committed to some version of the Principle of Plenitude (= No possibility remains unactualized through an infinity of time) which is applicable to perishable entities – i.e. the entities in the sublunary realm; (b) given that this is the case, then Aristotle’s drive to neutralize determinism, in the guise of fatalism (On Interpretation 9) and causal determinism (Metaphysics E. 3), suffers a serious setback; (c) Aristotle could have easily resolved this problem, had he become aware of it, by correcting the mistake committed in the argument of De Caelo A. 12 (281a28-b25).