Objection 1. It would seem that the universe of creatures, called the world, had no beginning, but existed from eternity. For everything which begins to exist, is a possible being before it exists: otherwise it would be impossible for it to exist. If therefore the world began to exist, it was a possible being before it began to exist. But possible being is matter, which is in potentiality to existence, which results from a form, and to non-existence, which results from privation of form. If therefore the world began to exist, matter must have existed before the world. But matter cannot exist without form: while the matter of the world with its form is the world. Therefore the world existed before it began to exist: which is impossible.
Objection 2. Further, nothing which has power to be always, sometimes is and sometimes is not; because so far as the power of a thing extends so long is exists. But every incorruptible thing has power to be always; for its power does not extend to any determinate time. Therefore no incorruptible thing sometimes is, and sometimes is not: but everything which has a beginning at some time is, and at some time is not; therefore no incorruptible thing begins to exist. But there are many incorruptible things in the world, as the celestial bodies and all intellectual substances. Therefore the world did not begin to exist.
Objection 3. Further, what is unbegotten has no beginning. But the Philosopher (Phys. i, text 82) proves that matter is unbegotten, and also (De Coelo et Mundo i, text 20) that the heaven is unbegotten. Therefore the universe did not begin to exist.
Objection 4. Further, a vacuum is where there is not a body, but there might be. But if the world began to exist, there was first no body where the body of the world now is; and yet it could be there, otherwise it would not be there now. Therefore before the world there was a vacuum; which is impossible.
Objection 5. Further, nothing begins anew to be moved except through either the mover or the thing moved being otherwise than it was before. But what is otherwise now than it was before, is moved. Therefore before every new movement there was a previous movement. Therefore movement always was; and therefore also the thing moved always was, because movement is only in a movable thing.
Objection 6. Further, every mover is either natural or voluntary. But neither begins to move except by some pre-existing movement. For nature always moves in the same manner: hence unless some change precede either in the nature of the mover, or in the movable thing, there cannot arise from the natural mover a movement which was not there before. And the will, without itself being changed, puts off doing what it proposes to do; but this can be only by some imagined change, at least on the part of time. Thus he who wills to make a house tomorrow, and not today, awaits something which will be tomorrow, but is not today; and at least awaits for today to pass, and for tomorrow to come; and this cannot be without change, because time is the measure of movement. Therefore it remains that before every new movement, there was a previous movement; and so the same conclusion follows as before.
Objection 7. Further, whatever is always in its beginning, and always in its end, cannot cease and cannot begin; because what begins is not in its end, and what ceases is not in its beginning. But time always is in its beginning and end, because there is no time except "now" which is the end of the past and the beginning of the future. Therefore time cannot begin or end, and consequently neither can movement, the measure of what is time.
Objection 8. Further, God is before the world either in the order of nature only, or also by duration. If in the order of nature only, therefore, since God is eternal, the world also is eternal. But if God is prior by duration; since what is prior and posterior in duration constitutes time, it follows that time existed before the world, which is impossible.
Objection 9. Further, if there is a sufficient cause, there is an effect; for a cause to which there is no effect is an imperfect cause, requiring something else to make the effect follow. But God is the sufficient cause of the world; being the final cause, by reason of His goodness, the exemplar cause by reason of His wisdom, and the efficient cause, by reason of His power as appears from the above (44, 2,3,4). Since therefore God is eternal, the world is also eternal.
Objection 10. Further, eternal action postulates an eternal effect. But the action of God is His substance, which is eternal. Therefore the world is eternal.
On the contrary, It is said (John 17:5), "Glorify Me, O Father, with Thyself with the glory which I had before the world was"; and (Proverbs 8:22), "The Lord possessed Me in the beginning of His ways, before He made anything from the beginning."
I answer that, Nothing except God can be eternal. And this statement is far from impossible to uphold: for it has been shown above (Question 19, Article 4) that the will of God is the cause of things. Therefore things are necessary, according as it is necessary for God to will them, since the necessity of the effect depends on the necessity of the cause (Metaph. v, text 6). Now it was shown above (Question 19, Article 3), that, absolutely speaking, it is not necessary that God should will anything except Himself. It is not therefore necessary for God to will that the world should always exist; but the world exists forasmuch as God wills it to exist, since the being of the world depends on the will of God, as on its cause. It is not therefore necessary for the world to be always; and hence it cannot be proved by demonstration.
Nor are Aristotle's reasons (Phys. viii) simply, but relatively, demonstrative--viz. in order to contradict the reasons of some of the ancients who asserted that the world began to exist in some quite impossible manner. This appears in three ways.
Firstly, because, both in Phys. viii and in De Coelo i, text 101, he premises some opinions, as those of Anaxagoras, Empedocles and Plato, and brings forward reasons to refute them.
Secondly, because wherever he speaks of this subject, he quotes the testimony of the ancients, which is not the way of a demonstrator, but of one persuading of what is probable.
Thirdly, because he expressly says (Topic. i, 9), that there are dialectical problems, about which we have nothing to say from reason, as, "whether the world is eternal."
Reply to Objection 1. Before the world existed it was possible for the world to be, not, indeed, according to a passive power which is matter, but according to the active power of God; and also, according as a thing is called absolutely possible, not in relation to any power, but from the sole habitude of the terms which are not repugnant to each other; in which sense possible is opposed to impossible, as appears from the Philosopher (Metaph. v, text 17).
Reply to Objection 2. Whatever has power always to be, from the fact of having that power, cannot sometimes be and sometimes not be; but before it received that power, it did not exist.
Hence this reason which is given by Aristotle (De Coelo i, text 120) does not prove simply that incorruptible things never began to exist; but that they did not begin by the natural mode whereby things generated and corruptible begin.
Reply to Objection 3. Aristotle (Phys. i, text 82) proves that matter is unbegotten from the fact that it has not a subject from which to derive its existence; and (De Coelo et Mundo i, text 20) he proves that heaven is ungenerated, forasmuch as it has no contrary from which to be generated. Hence it appears that no conclusion follows either way, except that matter and heaven did not begin by generation, as some said, especially about heaven. But we say that matter and heaven were produced into being by creation, as appears above (44, 1, ad 2).
Reply to Objection 4. The notion of a vacuum is not only "in which is nothing," but also implies a space capable of holding a body and in which there is not a body, as appears from Aristotle (Phys. iv, text 60). Whereas we hold that there was no place or space before the world was.
Reply to Objection 5. The first mover was always in the same state: but the first movable thing was not always so, because it began to be whereas hitherto it was not. This, however, was not through change, but by creation, which is not change, as said above (45, 2, as 2). Hence it is evident that this reason, which Aristotle gives (Phys. viii), is valid against those who admitted the existence of eternal movable things, but not eternal movement, as appears from the opinions of Anaxagoras and Empedocles. But we hold that from the moment that movable things began to exist movement also existed.
Reply to Objection 6. The first agent is a voluntary agent. And although He had the eternal will to produce some effect, yet He did not produce an eternal effect. Nor is it necessary for some change to be presupposed, not even on account of imaginary time. For we must take into consideration the difference between a particular agent, that presupposes something and produces something else, and the universal agent, who produces the whole. The particular agent produces the form, and presupposes the matter; and hence it is necessary that it introduce the form in due proportion into a suitable matter. Hence it is correct to say that it introduces the form into such matter, and not into another, on account of the different kinds of matter. But it is not correct to say so of God Who produces form and matter together: whereas it is correct to say of Him that He produces matter fitting to the form and to the end. Now, a particular agent presupposes time just as it presupposes matter. Hence it is correctly described as acting in time "after" and not in time "before," according to an imaginary succession of time after time. But the universal agent who produces the thing and time also, is not correctly described as acting now, and not before, according to an imaginary succession of time succeeding time, as if time were presupposed to His action; but He must be considered as giving time to His effect as much as and when He willed, and according to what was fitting to demonstrate His power. For the world leads more evidently to the knowledge of the divine creating power, if it was not always, than if it had always been; since everything which was not always manifestly has a cause; whereas this is not so manifest of what always was.
Reply to Objection 7. As is stated (Phys. iv, text 99), "before" and "after" belong to time, according as they are in movement. Hence beginning and end in time must be taken in the same way as in movement. Now, granted the eternity of movement, it is necessary that any given moment in movement be a beginning and an end of movement; which need not be if movement be a beginning. The same applies to the "now" of time. Thus it appears that the idea of the instant "now," as being always the beginning and end of time, presupposes the eternity of time and movement. Hence Aristotle brings forward this reason (Phys. viii, text 10) against those who asserted the eternity of time, but denied the eternity of movement.
Reply to Objection 8. God is prior to the world by priority of duration. But the word "prior" signifies priority not of time, but of eternity. Or we may say that it signifies the eternity of imaginary time, and not of time really existing; thus, when we say that above heaven there is nothing, the word "above" signifies only an imaginary place, according as it is possible to imagine other dimensions beyond those of the heavenly body.
Reply to Objection 9. As the effect follows from the cause that acts by nature, according to the mode of its form, so likewise it follows from the voluntary agent, according to the form preconceived and determined by the agent, as appears from what was said above (19, 4; 41, 2). Therefore, although God was from eternity the sufficient cause of the world, we should not say that the world was produced by Him, except as preordained by His will--that is, that it should have being after not being, in order more manifestly to declare its author.
Reply to Objection 10. Given the action, the effect follows according to the requirement of the form, which is the principle of action. But in agents acting by will, what is conceived and preordained is to be taken as the form, which is the principle of action. Therefore from the eternal action of God an eternal effect did not follow; but such an effect as God willed, an effect, to wit, which has being after not being.
Source: Summa Theologica (New Advent)