The question of the eternity of the world became a major issue in Paris with the reception of Aristotle in the following century. Siger of Brabant, in his early 1270s treatise De Aeternitate Mundi, articulates a relatively dry, uneditorialized case for the eternity of the world, substantially derived from Averroes. Metaphilosophically, Siger’s work represents a rising new genre in medieval thought: simply assuming the validity of its Aristotelian assumptions, the opusculum works out steady analysis without even bothering to seek any sort of concordia discordantium. (14) Siger makes no real attempt in his work to reconcile large-scale objections to his thesis; he merely demonstrates how it might be supported against limited counter-arguments. At the onset, he proposes two basic questions to be addressed. “The first question is whether the human species, and in general the species of all individuals began to exist only by way of the propagation of generable and corruptible things when it had no previous existence whatsoever." (15) The basic point here is that each species has its being and receives causation only as individuals. The second question shifts ground to reapproach the same issue from the standpoint of universals: “universals, just as they do not have existence in singulars, neither are so caused." (16) Again, Siger is essentially concerned to understand how a individual thing can begin to be.
To evaluate these questions, he proceeds to ask three more basic questions: how the human species is caused, how universals exist in singulars, and whether potentiality may proceed act in duration. (17) On the first point, he concludes that species may not be caused except by accidental generation. Thus, for Siger, species is not essentially generated.
It is not to be thought of as eternal and caused as if it existed abstracted from individuals. Nor . . . . in the sense that it exists in an eternally caused individual . . . . but rather because in the individuals of the human species one is generated before the other eternally, and the species has to be through and individual’s existing and being caused. (18)
At this point, Siger uses the analogy of Aristotle’s concept of time from Physics 4.13: Whatever is finite in quantity, while infinite in number, must be infinite. (19)
On the second question, he responds, following Aristotle, that universals exist in the mind. For Siger, the active intellect and the phantasm formed precede the act of the intelligible; accordingly, the universal does not get its universality from the nature causing the act of understanding. (20) In dealing with the third question, Siger addresses “whether act precedes potentiality in time or potentiality the act.” (21) Following Aristotle in Metaphysics 9.8, he argues “that in a being which is the same in number proceeding from potency to act, potency proceeds act; but that, nevertheless, before the being in potency there is another of the same species in act, educing from it potency to act.” (22) Thus, Siger winds up affirming that act thus effectively precedes potency, as all beings go into actuality from potentiality through an agent existing in act. “Hence it is that in any given being in potentiality to some act, the act of the species in a certain way, although not entirely for the same reason, precedes that potentiality in time; not however, in any given being in act does potentiality from which it proceeds to act, proceed." (23) This would prove the eternity of the world, for “the first mover leading into act all being in potentiality does not precede in time the being in potentiality, since the being in potentiality is regarded in the rank of prime matter. For, just as God always exists, according to Aristotle, so also does the potential man, since he is regarded as in prime matter." (24) What Siger seems to be saying this that the world must consist of parts in eternal act and potency, the former acting upon the latter and the latter allowing the former to act.
Boukowksi suggest that Siger, in addressing the causation of the world here in De Aeternitate Mundi and elsewhere in his commentaries on Aristotle, is merely presenting probably, naturalistic arguments based on logical extrapolation from imperfect philosophers. According to this interpretation, he dispassionately expounds the implications of an unmoved first cause as envisioned by Aristotle and Averroes; while implicitly accepting the rappochment of philosophy and Law made explict in Boethius of Dacia. Within Siger’s philosophical account, then, the first cause requires an eternal effect, and thus generates the world eternally. The natural explanation is adequate, on its own level of knowledge; faith is left free to affirm something quite separate, on another level. (25) Here again, Siger plays the proto-Renaissance “humanist,” more interested in the thought than whether it counts.
Probably a year after Siger’s writing, Thomas Aquinas treated the problem in a treatise of his own, his own De Aeternitate Mundi. Whereas Siger’s treatise is short on “philosophy of religion” commentary, Aquinas’ libellum exudes it. For Aquinas, the eternity of the world problem represented a potential peril ready to taint the critical, adaptive use of Aristotle with the taint of impiety—a fear later given form in the Condemnations of 1277. From the onset of his opusculum, he seeks clarify that it is one thing to investigate the mode of creative causality and, in the context, to question whether the world could be coeternal with its cause; and quite another thing to question divine causality categorically by asserting the existence of an uncaused, eternal, autonomous universe. This possibility is obvious to Aquinas simply on the basis of the divine attributes: “all parties are agreed that God could make something that always existed, because of the fact that his power is infinite.” (26) Aquinas deems it improbable that God should cause the world eternally, but demands the investigation of their logical compatibility rather than an outright dismissal. “Since God’s omnipotence surpasses all understanding and power, anyone who asserts that something which in intelligible among creatures cannot be made by God, openly disparages God’s omnipotence." (27) At bottom, the Angelic Doctor seems to be arguing for free inquiry, fides quarens intellectum.
In the first place, Aquinas finds that no logical contradiction forbids an agent not to precede an effect in duration. Causes acting instantaneously, such as fire or motion demonstrate this. Furthermore, only the will seems able to slow the causal agent, so that it need precede its effect in duration. Secondarily, he seeks to explore whether further contradiction is involved for a created thing to have never been without existence. Citing Anselm in Monologium 8, wherein that doctor uses the example of the man saddened without cause as being saddened by nothing, he immediately finds that “no order of duration is establish between what was made and nothing, as though what is made would first have to be nothing, and afterwards have to something.” (28) Continuing further, Aquinas drives a distinction between order of duration and order of nature, suggesting that the relation between uncreating and created being is analogous to that between universal and particulars. “For our position is not that, if the creature had always existed, it was nothing at some time. We maintain that its nature is such that it would be nothing if it were left to itself.” (30) A good example of this is the sun, which without duration makes the air luminous. A further example, borrowed from De Civitate Dei 10.31, is the impression made by a foot planted eternally in dust. Having raised Augustine, Thomas then proceeds to cite several esteemed philosophers, including Hugh of St. Victor and Boethius ipse: the created world, while never without existence, does not exist in the same sense of an eternal present as God, being mutable.
While Aquinas fails to find the philosophical doctrine of the eternity of the world on purely naturalistic grounds, on theological grounds, he finds no obligation to believe in an eternally existing world. God’s will being the cause of all things, “God is under no compulsion to will anything except himself. All we can say is the world is eternal if God wills it to be so, since the existence of the world depends upon God’s will as a cause.” (31) However, revelation demands that we deny the eternity of the world: “I hold by faith alone that the world has not existed forever; this truth cannot be proven demonstrably,” any more than may the mystery of the Trinity. Simply put, “the inception of the world cannot be demonstrated from the standpoint of the world itself. (32) At bottom, for Aquinas, the problem with the Aristotelian argument for the eternity of the world is that it conflates efficient causation and creation: “The first mover was always in the same state, but the first movable things was not always been in the same state, because it began to exist, whereas previously it had not existed. However, this was brought about not by change but by creation, which is not change.” (33) Again, “[Aristotle’s] argument did not proceed from the consideration which regards the universe as taking its origin from God, but from the consideration that an agent which begins to act must itself be moved; in other words, he was thinking of a particular cause, not the universal cause.” (33)