Why, though, should everything finite [in power] be perishable? And why should everything perishable perish? It is here that the temporalized notion of Plenitude enters into play, and here also that Averroes' allusion to De caelo as the textual grounds for Philoponus' argumentation becomes of interest. In De caelo 1, ch. 12 Aristotle presents what seems a rather curious argument for the impossibility of the world's ever not existing. Aristotle has already previously argued for his view that the kosmos, the rationally ordered universe is eternal: it has always existed and it will always exist (De caelo 1, chs. 9-11). Given this, Aristotle now claims, even conceiving of the world as possibly not existing is self-defeating, if not self-contradictory: for
...if anything which exists for an infinite time is perishable, it will have the capacity (dynamis) of not being. Now if it exists for infinite time let this capacity be actualized; and it will be in actuality at once existent and non-existent.10
The conclusion is manifestly impossible: Aristotle promptly concludes that, contrary to the initial assumption, "anything which always exists is imperishable without qualification".11 Clearly flawed as the argument may be in our eyes,12 it is important to recognize that its theoretical background somehow includes the idea of all possibilities being realized within a single universal history.13 This is tantamount to temporalized Plenitude in its strictest, most particularized form. The possibility of some thing's perishing equals its sometime perishing, which again precludes predicating eternity of that thing. And this is just what Averroes points out in the Metaphysics commentary passage we have quoted.
Unqualified temporalized Plenitude as applied to singular limited beings is such an absurd proposition that even Aristotle went out of his way to avoid it and its manifestly deterministic conclusions. But he did apparently believe in the principle's validity with regard to eternal beings and attributes proper to them; as the matter is put in the Physics, "in the case of eternal things what may be is".14 Interpreters of Aristotle ranging from Alexander of Aphrodisias in Antiquity to Thomas Aquinas in high scholasticism tried to account for the fact in some way.1 The theory of potentiality and actuality expounded in the Metaphysics came in handy here, as it serves to bridge the gap between finitude and infinity, on the one hand, and perishability and imperishability on the other. Change in mature Aristotelian theory is analyzable in terms of interchanging states of potentiality and actuality. Whenever something becomes something, it both exercises (actualizes) a certain natural capacity in the very act of changing so and in the end result brings about another actualization, that of the capacity for becoming so. For contraries and contradictories, the periods of being actual and potential complement each other: whenever something is actually sitting, it is potentially standing and vice versa. Rest comes to complement any period of limited actual motion; so also, when the actuality of a thing's existence ceases, the privation of this existence, until then lying dormant, is in a sense "activated".
Now, an infinite time frame exceeds any finite time that a finite potency can actualize some particular activity; it seems natural to posit an opposite capacity that is actualized in the time "left over". And this, it seems, is precisely what Philoponus suggests will happen to the heavens, once their finite powers have run their course. Shrewdly equivocating between the dynameis of De caelo and those of the Physics and of the Metaphysics, Philoponus takes it that the potencies inhering in the heavens can only be active for a finite time. Once this is granted, an appeal to temporal Plenitude is enough to ensure that the complementary possibility for rest (or destruction, should the De Caelo context also be taken into account) cannot go infinitely unheeded—not even if the First Mover is brought in. In a quotation from Simplicius, we find Philoponus insisting that there exists in the heavens a possibility (in Simplicius' Greek account, we read logos) for destruction to complement their now-actual potentiality for existence. Philoponus urges the Aristotelian exegete to acknowledge that, divine clauses aside, the world should be viewed as naturally finite in extension, both a parte ante and a parte post:
For our question at the moment is this: What is the natural definition of each thing? and not: What is added to something by the transcendent cause? In consequence, although one might agree that the heavens do not perish because they are held together by the will of God, it is emphatically not the case as well that in virtue of their own nature they will not accept the formal character (logos) of destruction. But what is of the formal character to be destroyed is not infinite in capacity.16 For how could it be that this formal character of destruction which is comprehended by our mind will not at some time become actual?17
The last question is purely rhetorical: Philoponus imagines the naturalist Aristotelian to reply with him that there is really no way the actualization of any genuine possibility can be avoided, given infinite time. Philoponus' argument requires the acceptance of temporal Plenitude for it to even begin to make sense:18 but once granted, the argument reads in effect like an inversion of Aristotle's in De caelo. Philoponus' first premise is that the heaven on account of its own finitude is perishable (mutatis mutandis, this is the only element taken over from Aristotle's own infinite power argument). Next, let it be assumed that an infinite power were to be supernaturally meted out (by God, as was the Neoplatonist explanation from Proclus onwards) to counter this destruction, so that the heavens de facto become eternal. But then, assuming that the potentiality for the heavens' perishing must needs be actualized, the heavens at some point in infinite time both exist and do not exist: which is impossible. The assumption of an infinite prolongation cannot be countenanced as long as the original premise about the natural finitude of all bodies is retained; ergo, the heavens perish in actuality; ergo, they have been generated.19
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