DE AETERNITATE MUNDI
On the Eternity of the World From http://www.fordham.edu/halsall/basis/aquinas-eternity.html
Translation � 1991, 1997 by Robert T. Miller 
reformated by Joseph Kenny, O.P.
Let us assume, in accordance with the Catholic faith, that the world had a beginning in time. The question still arises whether the world could have always existed, and to explain the truth of this matter, we should first distinguish where we agree with our opponents from where we disagree with them. If someone holds that something besides God could have always existed, in the sense that there could be something always existing and yet not made by God, then we differ with him: such an abominable error is contrary not only to the faith but also to the teachings of the philosophers, who confess and prove that everything that in any way exists cannot exist unless it be caused by him who supremely and most truly has existence. However, someone may hold that there has always existed something that, nevertheless, had been wholly caused by God, and thus we ought to determine whether this position is tenable.
If it be impossible that something caused by God has always existed, it will be so either because God could not make something that has always existed or because such a thing could not be made, regardless of God's ability to make it.  As to the first, all parties agree that, in view of his infinite power, God could have made something that has always existed.  It remains to be seen, therefore, whether something that has always existed can be made.
If such a thing cannot be made, the impossibility will arise for one of two reasons: either because of an absence of a passive potentiality or because of some contradiction between the ideas involved.  In regard to the first, notice that before an angel is made, we may say, in a certain manner of speaking, that the angel cannot be made,  since no passive potentiality precedes its being, for an angel is not made from pre-existing matter. Nevertheless, God was able to make the angel, and he was able to cause the angel to be made, for God made it, and it was made. Therefore, if we understand "being made" or "being caused" as implying the pre-existence of a passive potentiality, then it should to be conceded, according to faith, that something caused cannot always exist, for it would then follow that a passive potentiality has always existed, and this is heretical. But since a passive potentiality need not precede in time whatever God may make, it does not follow that God could not have made something that has always existed.
In regard to the second, someone may hold that something that has always existed cannot be made because such a thing is self-contradictory, just as an affirmation and a denial cannot be made simultaneously true. Still, some people say that God can even make self-contradictories things, while others say God cannot make such things, for such things are actually nothing. Clearly, God cannot make such things come to be, for the assumption that such a thing exists immediately refutes itself. Nevertheless, if we allow that God can make such things come to be, the position is not heretical, though I believe it is false, just as the proposition that the past did not occur is false, about which Augustine says (XXVI Contra Faustum cap. 5), "Anyone who says, 'If God is omnipotent, let him make what has happened not to have happened,' does not realize that he is saying, 'If God is omnipotent, let him make true things false insofar as they are true.'" [PL 42, 481.] Nevertheless, certain great men have piously maintained that God can make past events not to have happened, and this was not reputed to be heretical.
We thus ought to determine whether there is any contradiction between these two ideas, namely, to be made by God and to have always existed. And, whatever may be the truth of this matter, it will not be heretical to say that God can make something created by him to have always existed, though I believe that if there were a contradiction involved in asserting this, the assertion would be false. However, if there is no contradiction involved, then it is neither false nor impossible that God could have made something that has always existed, and it will be an error to say otherwise. For, if there is no contradiction, we ought to admit that God could have made something that has always existed, for it would be clearly derogatory to the divine omnipotence, which exceeds every thought and power, to say that we creatures can conceive of something that God is unable to make. (Nor are sins an instance to the contrary, for, considered in themselves, they are nothing.) In this, therefore, the entire question consists: whether to be wholly created by God and not to have a beginning in time are contradictory terms.
That they are not contradictory can be shown as follows. If they are contradictory, this is for one or both of these two reasons: either because the agent cause must precede the effect in time, or because non-being must precede the effect in time, for we say that what God creates comes to be out of nothing.
First, we should show that it is not necessary that an agent cause, in this case God, precede in time that which he causes, if he should so will. This can be shown in several ways. First, no cause instantaneously producing its effect necessarily precedes the effect in time. God, however, is a cause that produces effects not through motion but instantaneously. Therefore, it is not necessary that he precede his effects in time. The first premise is proved inductively from all instantaneous changes, as, for example, with illumination and other such things. But the premise may be proved by reason as well.
For, at whatever instant a thing exists, at that instant it can begin to act, as is clear in the case of all things that come to be by generation: in the very instant at which there is fire, the fire heats. But in an instantaneous action, the beginning and the end of the action are simultaneous, indeed identical, as is clear in the case of all indivisible things. Hence, at whatever moment an agent instantaneously producing an effect exists, the end of its action can exist as well. The end of the action, however, is simultaneous with the thing made. Therefore, there is no contradiction if we suppose that a cause instantaneously producing an effect does not precede its effect in time. A contradiction does obtain if the cause involved is one that produces its effects through motion, for the beginning of the motion precedes in time the end of the motion. Since people are accustomed to considering the type of cause that produces effects through motion, they do not easily grasp that an agent cause may fail to precede its effect in time, and so, having limited experience, they easily make a false generalization.
Nor can the conclusion be avoided by saying that God is an agent cause that acts voluntarily, for neither the will nor the voluntary agent need precede its effect in time, unless the agent cause acts from deliberation, which we take to be absent in God.
Further, a cause that produces the whole substance of a thing does not, in producing a whole substance, act in a less perfect way than does a cause that produces just a form in producing the form. On the contrary, it acts in a much more perfect way, since it does not act by educing from the potentiality of matter, as do causes that merely produce forms. However, some causes that produce just forms are such that, whenever the cause exists, the form produced by it exists as well, as is clear in the case of illumination by the sun. Therefore, much more can God, who produces the whole substance of things, make something caused by him exist whenever he himself exists.
Further, if, granted a cause, its effect does not immediately exist as well, this can only be because something complementary to that cause is lacking: the complete cause and the thing caused are simultaneous. God, however, never lacks any kind of complementary cause in order to produce an effect. Therefore, at any instant at which God exists, so too can his effects, and thus God need not precede his effects in time.
Further, the will of the voluntary agent in no way diminishes his power, and this is especially true with God. But all those who try to answer the arguments of Aristotle (who held that something caused by God had always existed, since like always makes like)  say that the conclusion would follow if God were not a voluntary agent. Therefore, allowing that God is a voluntary agent, it still follows that he can make something that he has made never fail to exist. Thus, although God cannot make contradictories true, we have shown that there is no contradiction in saying that an agent cause does not precede its effect in time.
It remains to be seen, then, whether there is a contradiction in saying that something made has always existed, on the grounds that it may be necessary that its non-being precede it in time, for we say that it is made out of nothing. But that there is no contradiction here is shown by Anselm in his explanation of what it means to say that a creature is made out of nothing. He says (Monologion cap. 8), "The third sense in which we can say that something is made out of nothing is this: we understand that something is made, but that there is not something from which it is made. In a similar way, we say that someone who is sad without reason is sad about nothing. We can thus say that all things, except the Supreme Being, are made by him out of nothing in the sense that they are not made out of anything, and no absurdity results." On this understanding of the phrase "out of nothing," therefore, no temporal priority of non-being to being is posited, as there would be if there were first nothing and then later something.
Further, let us even suppose that the preposition "out of" imports some affirmative order of non-being to being, as if the proposition that the creature is made out of nothing meant that the creature is made after nothing. Then this expression "after" certainly implies order, but order is of two kinds: order of time and order of nature. If, therefore, the proper and the particular does not follow from the common and the universal, it will not necessarily follow that, because the creature is made after nothing, non-being is temporally prior to the being of the creature. Rather, it suffices that non-being be prior to being by nature. Now, whatever naturally pertains to something in itself is prior to what that thing only receives from another. A creature does not have being, however, except from another, for, considered in itself, every creature is nothing, and thus, with respect to the creature, non-being is prior to being by nature. Nor does it follow from the creature's always having existed that its being and non-being are ever simultaneous, as if the creature always existed but at some time nothing existed, for the priority is not one of time. Rather, the argument merely requires that the nature of the creature is such that, if the creature were left to itself, it would be nothing. For example, if we should say that the air has always been illuminated by the sun, it would be right to say that the air has always been made lucid by the sun. Thus, since anything that comes to be such-and-such comes to be such-and-such from being not such-and-such, we say that the air is made lucid from being non-lucid, or opaque, not because the air was once non-lucid or opaque, but because the air would be opaque if the sun did not illuminate it. This is clearly the case with the stars and those celestial bodies that are always illuminated by the sun.
Thus it is clear that there is no contradiction in saying that something made by God has always existed. Indeed, if there were some contradiction, it would be amazing that Augustine failed to see it, for exposing such a contradiction would be a most effective way of proving that the world is not eternal, and although Augustine offers many arguments against the eternity of the world in XI and XII De Civitate Dei, he never argues that his opponents' view is contradictory. On the contrary, Augustine seems to hint that there is no contradiction involved. Thus, speaking of the Platonists, he says (X De Civitate Dei cap. 31), "They somehow contemplate a beginning in causation rather than a beginning in time. Imagine, they say, a foot that has been in dust since eternity: a footprint has always been beneath it, and nobody would doubt that the footprint was made by the pressure of the foot. Though neither is prior in time to the other, yet one is made by the other. Likewise, they say, the world and the gods in it have always existed, just as he who made them always existed; yet nevertheless, they were made." [PL 41, 311] Nor does Augustine ever say that this cannot be understood; rather, he proceeds against the Platonists in a totally different way. He says (XI De Civitate Dei cap. 4), "Those, however, who admit that the world was made by God but nevertheless want to hold that the world has a beginning in creation but not in time, so that, in some scarcely intelligible way, it has always been made by God, think that they are defending God against a charge of casual rashness." [PL 41, 319]  Their position is difficult to understand, however, only for the reason given above in the first argument.
How remarkable it would be that even the most noble of philosophers failed to see a contradiction in the idea that something made by God has always existed. Speaking against the Platonists, Augustine says (XI De Civitate Dei cap. 5), "Here we are contending with those who agree with us that God is the Creator of all bodies and all natures except himself," [PL 41, 320] and then, again about the Platonists, he adds (XI De Civitate Dei cap. 5), "These philosophers surpassed the rest in nobility and authority." [PL 41, 321] Augustine said this even after diligently considering their position that the world has always existed, for they nevertheless thought that it was made by God, and they saw no contradiction between these two ideas. Therefore, those who so subtly perceive the contradiction are solitary men, and with these does wisdom arise. 
Still, since certain authorities seem to argue on the side of such men, we ought to show that they base themselves on a weak foundation. Damascene says (I De Fide Orthodoxa cap. 8), "What is made out of nothing is by nature not such that it is coeternal to what has no causal principle and always exists." [PG 94, 814B] Likewise, Hugh of St. Victor says (De Sacramentis I-1 cap. 1), "The ineffable omnipotent power could not have anything coeternal beyond itself that would help it in making." [PL 176, 187B]
But the position of these and similar authorities is made clear by Boethius, who says (V De Consolatione prosa 6), "When some people hear that Plato thought this world neither had a beginning in time nor will ever have an end, they mistakenly conclude that the created world is coeternal with the Creator. However, to be led through the endless life Plato attributes to the world is one thing; to embrace simultaneously the whole presence of endless life is quite another, and it is this latter that is proper to the divine mind." [PL 63, 859B] Thus it does not follow, as some people object, that a creature, even if it had always existed, would be equal to God in duration. For, if "eternal" be understood in this sense, nothing can in any way be coeternal with God, for nothing but God is immutable. As Augustine says (XII De Civitate Dei cap. 15),  "Time, since it passes away by its mutability, cannot be coeternal with immutable eternity. Thus, even if the immortality of the angels does not pass away in time (it is neither past, as if it did not exist now; nor is it future, as if it did not yet exist), nevertheless, the angels' motions, by which moments of time are carried along from the future into the past, pass away. Therefore, angels cannot be coeternal with the Creator, in whose motion there is nothing which has been that is not now, nor anything which will later be that is not already." [PL 41, 364-365] Likewise, Augustine says (VIII Super Genesis ad Litteram cap. 23), "Since the nature of the Trinity is wholly unchangeable, it is eternal in such a way that nothing can be coeternal with it," [PL 34, 389] and he uses words to the same effect in XI Confessionum as well. 
Those who try to prove that the world could not have always existed even adduce arguments that the philosophers have considered and solved. Chief among these is the argument from the infinity of souls: if the world had always existed, these people argue, there would necessarily be an infinite number of souls. But this argument is not to the point, for God could have made the world without making men or creatures with souls, or he could have made men when in fact he did make them, even if he had made the rest of the world from eternity. In either case, an infinite number of souls would not remain after the bodies had passed away. Furthermore, it has not yet been demonstrated that God cannot cause an infinite number of things to exist simultaneously.
There are other arguments adduced as well, but I refrain from answering them at present, either because they have been suitably answered elsewhere, or because they are so weak that their very weakness lends probability to the opposing view.
 This translation follows the Leonine Edition of Aquinas's works, vol. 43 Sancti Thomae De Aquino Opera Omnia 85-89 (Rome 1976).
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 Aquinas means that the impossibility may be thought to arise either on the part of God, as if he were unable to make such a thing for lack of power, or on the part of the thing, as if such a thing could not be made because it lacks a pre-existing passive potentiality or because it is self-contradictory.
 That is, on the condition that such a thing can be made. In other words, all sides agree that the impossibility of something having always existed, if such there be, does not arise from some lack of power in God.
 That is, between "always having existed" and "having been made."
 In the sense that there was nothing existing before the angel that would become the angel, as the brass to be made into a statue exists before the statue and becomes the statue.
 See II De Generatione et Corruptione cap. 10, 336a 27-28.
 PL 41, 319. In the Leonine Edition, Aquinas does not quote the predicate of the independent clause; it does appear in the Parma Edition, and I have chosen to supply it.
 Said ironically, the sentence is quite out of character for Aquinas, who courteously conducted the bitterest disputations. Here he is probably alluding to the Vulgate text of Job 12:2, in which Job says, "You are solitary men, and with you wisdom shall die." The difference between "arises" (oritur) and "shall die" (morietur) is small.
 So in Aquinas. The chapter divisions in De Civitate Dei are, at this point, somewhat unclear, and, as the editors of the Leonine Edition suggest, the quoted text is probably from cap. 16. In any event, the quoted material appears at PL 41, 364-365.
 See XI Confessionum cap. 30. PL 32, 826.
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Paul Halsall May 1997