Lecture 5 — An eternal immovable substance must exist

Since there are three kinds of substance (1069a30), two of which are physical and one immovable, concerning the latter, it is necessary to affirm that an eternal immovable substance must exist. For substances are the primary kind of beings, and if all of them are perishable, all things are perishable.

But it is impossible either that motion should have come to be or that it should perish, for it always existed. The same is true of time, for there cannot be a before and an after if there is no time. Motion is continuous, then, in the sense that time is, for time is either the same as motion or a property of it. Now the only continuous motion is that which pertains to place, and of this, only that which is circular.

But, even if there is something which is capable of imparting or producing motion but is not actually doing so, motion will still not exist; for that which has a potency may possibly not exercise it.

Hence, nothing is to be gained if we invent certain eternal substances, as do those who posit the separate forms, unless there is some principle among them which is capable of causing change (988a34). This is not sufficient, then, nor is another substance besides the separate forms sufficient; for if it does not act, there will be no eternal motion.

And even if it does act, this will still not be sufficient, if its essence is a potency; for there will be no eternal motion, since what is potential may possibly not be. Hence, there must be a principle of the kind whose substance is an act.

Further, such substances must also be immaterial; for they must be eternal if anything else is. Hence they are acts.

2488. After having shown what the principles of sensible substances are, here the Philosopher begins to establish the truth about the immovable substances, which are separate from matter. This topic is divided into two parts.

First (1071b3; [2488]), he treats substances of this sort by giving his own opinion.

Second, he treats them by giving the opinions of other thinkers. He does this in the following book, at concerning the substance of sensible things (1076a8). (This book is not included in Thomas’s commentary.)

The first part is divided into two members.

First, he proves that there is a substance that is eternal, immovable, and separate from matter.

Second (1072a26; [2519]), he investigates the attributes of this substance, at now the first mover.

In regard to the first, he does three things.

First, he proves that an eternal substance must exist.

Second (1071b22; [2500]), he deals with a question arising from the foregoing discussion, at there is a difficulty, however.

Third (1072a7; [2508]), from the answer given to the question that was raised, he proceeds to clarify a truth previously established, at hence, Chaos or Night.

In regard to the first, he does two things.

First, he shows that it is necessary to posit an eternal substance.

Second (1071b12; [2492]), he shows what kind of substance it must be, at but even if there is.

He accordingly first says (1071b3) that it has been pointed out above (1069a30; [2424]) that there are three kinds of substances. Two of these are natural substances, because they undergo motion: one is eternal (the heavens), and the other is perishable (plants and animals). And besides these, there is a third kind, which is immovable and not natural, and of this kind of substance, it is now necessary to speak.

With a view to investigating this kind of substance, it is first necessary to prove that an eternal immovable substance must exist. He proceeds as follows.

2489. Substances are the primary kind of beings, as has been shown above (1069a19; [2417–23]), and when primary things are destroyed, none of the others remain. Therefore, if no substance is eternal but all are perishable, it follows that nothing is eternal, but that all things are perishable, that is, they do not always exist. But this is impossible. Hence there must be an eternal substance.

2490. That it is impossible for nothing to be eternal he proves from the fact that motion cannot have come to be or perish, that is, it cannot have come to be anew or at some time totally cease to be. For it has been shown in Physics 8 that motion is eternal without qualification. It also seems impossible that time should not be eternal; for if time began to be at some time or will cease to be at some time, it would follow that prior to time there was the non-being of time, and also that there will be time after the non-being of time. But this seems to be impossible, because there could be no before or after if time did not exist, since time is nothing else than the measure of before and after in motion. Thus it would follow that time existed before it began to be, and that it will exist after it ceases to be. Hence it seems that time must be eternal.

2491. And if time is continuous and eternal, motion must be continuous and eternal, because either motion and time are the same thing, as some claimed, or time is a property of motion, as is really the case. For time is the measure of motion, as is evident in Physics 4. However, it must not be thought that every motion can be eternal and continuous, since this can be true only of local motion; and among local motions this is true only of circular motion, as is proved in Physics 8.

2492. But even if (1071b12). Then he shows what kind of substance this eternal substance must be, and in regard to this, he does three things.

First, he shows that, in order to account for the eternity of motion, it is necessary to posit an eternal substance that is always moving or acting. He says that, since it is necessary, on the assumption that motion is eternal, that there be an eternal substance that is capable of imparting or producing motion, it is also necessary that this be a mover or agent that is always acting, for if it were capable of imparting or producing motion, that is, if it had the power to produce or cause motion, and was not actually doing so, it would follow that there would be no actual motion. For that which has the power of causing motion may possibly not be causing it, since that which has the power of acting may possibly not act; thus motion would not be eternal. Assuming, then, that motion is eternal, it is necessary to posit an eternal substance that is actually moving or acting.

2493. Next, he concludes from this that nothing is to be gained by accepting the opinion of Plato, who posited eternal substances, since this is not sufficient to account for the eternity of motion. For the assumption that there are certain separate and eternal substances is not sufficient to account for this unless there is some principle among them which can cause change, but this does not seem to fit the separate forms. For Plato claimed that the separate forms are nothing else than universals existing apart from matter. But universals as such do not cause motion, for every active or motive principle is a singular thing, as has been pointed out above (1071a17; [2482]). Neither the separate forms, then, nor any other separate substances besides the forms, such as the separate mathematical entities posited by some, are sufficient to account for the eternity of motion, because even the objects of mathematics as such are not principles of motion. And if there is no eternal active substance, there will be no eternal motion, because the principle of motion is an eternal substance, which is a mover or agent.

2494. And even if (1071b17). Second, he shows that, in order for motion to be eternal, it is necessary not only that an eternal substance exist, which is a mover or agent, but also that its essence be an act. Hence he says that the eternity of motion is not adequately accounted for even if it is supposed that an eternal substance does act, yet is potential in essence. For example, it would not be sufficient to hold that the first principles are fire or water, as the ancient natural philosophers did, because then motion could not be eternal. For if a mover is such that its essence contains potency, it can possibly not be, because whatever is in potency may possibly not be. Hence, it would be possible for motion not to be, and so it would not be necessary and eternal. Therefore, it follows that there must be a first principle of motion of the sort whose essence is not in potency, but is only an act.

2495. Further, such substances (1071b20). Third, he further concludes that this kind of substance must be immaterial. He says that it also follows from the foregoing (1071b3–1071b17; [2488–94]) that substances of this kind, which are the principles of eternal motion, must be free from matter, for matter is in potency. Therefore, they must be eternal if something else is eternal, like motion and time. Thus it follows that they are in act.

2496. He finally concludes in this way because of the question that he will raise next. From this reasoning, then, it is evident that here Aristotle firmly thought and believed that motion and time must be eternal; otherwise, he would not have based his plan of investigating immaterial substances on this conviction.

2497. Yet it should be noted that the arguments that he introduces in Physics 8, which he assumes as the basis of his procedure here, are not demonstrations in the strict sense but only probable arguments—unless perhaps they are arguments against the positions of the ancient natural philosophers regarding the beginning of motion, inasmuch as he aims to destroy these positions.

2498. And aside from the other arguments which he does not touch upon here, it is evident that the argument that he does give here to prove that time is eternal is not demonstrative. For if we suppose that, at some moment, time began to be, it is not necessary to assume a prior moment except in imaginary time; just as when we say that there is no body outside of the heavens, what we mean by “outside” is merely an imaginary something. Hence, just as it is not necessary to posit some place outside of the heavens, even though “outside” seems to signify place, so too neither is it necessary that there be a time before time began to be, or a time after time will cease to be, even though “before” and “after” signify time.

2499. But, even if the arguments that prove that motion and time are eternal are not demonstrative and necessarily conclusive, still the things that are proved about the eternity and immateriality of the first substance necessarily follow. For even if the world were not eternal, it would still have to be brought into being by something that has prior existence. And if this cause were not eternal, it too would have to be produced by something else. But since there cannot be an infinite series, as has been proved in book 2 (994a11; [301–4]), it is necessary to posit an eternal substance whose essence contains no potency and is therefore immaterial.