Lecture 6 — Eternal motion requires an eternal mover

There is a difficulty, however; for it seems that, while everything that is acting is able to act, not everything that is able to act is acting. Thus potency is prior.

But if this is so, no beings will exist; for everything may be capable of being, but still not be. And if we take what the theologians say, who generate everything from night, or what the philosophers of nature say, who affirm that all things were together, they express the same impossible view. For how will things be moved, if there is no actual cause? Matter will not move itself, but technical knowledge will move it; nor will menstrual blood or earth move themselves, but semen or seed will move them.

This is the reason why some men, such as Leucippus and Plato, posit something which is always actual, for they say that motion always exists. But they do not say why it exists, or what it is, or how this is so, or what its cause is. For nothing is moved by chance, but there must always be something existing which moves it.

Now things are moved in one way by nature, and in another by force or by mind or by some other agent. What kind of motion, then, is prior? For this makes the greatest difference. Plato cannot explain what it is that he sometimes thinks is the source of motion, namely, what moves itself. According to him, the soul is later than motion and simultaneous with the heavens.

Now, to think that potency is prior to act is in one sense right and in another not, and we have explained how this is so (1071b22).

That act is prior is affirmed by Anaxagoras (for mind is an act), and by Empedocles in his theory of love and strife (984b32), and by those who say that motion always existed, as Plato and Leucippus.

Hence Chaos or Night did not exist for an infinite time, but the same things have always existed, either in a cycle or in some other way, granted that act is prior to potency.

Therefore, if something is always moved in the same cycle, there must be something which always continues to act in the same way. But if there is to be generation and destruction, there must be something else which acts in different ways. Hence this must act in one way of itself and in another way in virtue of something else, namely, in virtue either of some third agent or of the first.

Now, it must be in virtue of the first, for this is the cause both of the second and of the third. The first is preferable, then, for it was the cause of that whose being is always to be the same, and something else was the cause of that whose being is to be different; and obviously both of these account for eternal diversity.

Therefore, if motion always exhibits these characteristics, why is it necessary to look for other principles?

And since this is a possible account of the matter, and if this is not so, all things will come from Night (1071b25) or all things were together (1071b25) or something comes from non-being (1069b26), these difficulties are solved.

And there is something which is always being moved with an unceasing motion, and this is circular motion. This is evident not only in theory, but in fact, and for this reason, the first heaven will be eternal.

Therefore, there is also something which causes it to move. And since that which is moved and causes motion is intermediate, there must be something which causes motion and is unmoved, which is eternal and both a substance and an act.

2500. He raises a question about a point already dealt with. The question is whether act is prior absolutely to potency such that the first principle of things can be held to be one whose substance is act. In regard to this, he does three things.

First (1071b22; [2500]), he gives an argument to show what is false, namely, that potency is prior absolutely to act.

Second (1071b25; [2501]), he argues on the other side of the question, at but if this is so.

Third (1072a3; [2506]), he answers the question, at now, to think.

He accordingly first says (1071b22) that it has been pointed out that an eternal substance is an act, although there is a difficulty regarding this. For potency seems to be prior to act, since one thing is prior to another when the sequence of their being cannot be reversed (1019a2; [950]). Now potency seems to be related to act in this way, because everything that is acting seems to be able to act, but not everything that is able to act is acting. Thus it seems that potency is prior to act.

2501. But if this is so (1071b25). Then he argues on the opposite side of the question, and in regard to this, he does two things.

First, he gives an argument reducing the counter-position to absurdity. He says that, if potency is prior absolutely to act, it follows that, at some time, nothing may exist; for the contingent is what can come to be but has not yet done so. Hence, if the primary beings are potential, it follows that they do not exist actually; and so no other being will exist.

2502. This can be taken in two ways.

First, according to the opinion of certain of the ancients who were called the theological poets, such as Orpheus and certain others, who claimed that the world is generated from night, that is, from a simple preexistent privation.

Second, according to the later physicists—namely the philosophers of nature and their followers—who, when they saw that nothing comes from nothing in the natural world, claimed that all things were together in a kind of mixture, which they called Chaos (as did Anaxagoras, for example), and thus held that all things exist potentially and not actually.

2503. But, whether this position is stated in the former or in the latter way, the same impossible conclusion follows, provided that potency is prior absolutely to act. For those things that are in potency only, or which come entirely under privation, or belong to some confused mass, cannot be moved so as to be brought to act unless there is some moving cause that is existing actually. For in things made by art, the matter does not move itself, but an agent moves it, namely, technical knowledge, or art. Neither does the menstrual blood, which is the matter from which an animal is generated, move itself, but semen, that is, the sperm of the animal, moves it. Nor does earth, which is the material from which plants are generated, move itself, but the seed, namely, the seeds of plants, move it.

2504. This is the reason (1071b31). Second, he shows how some of the philosophers of nature agreed with this argument. He says that this is the reason why some philosophers—Leucippus, the companion of Democritus, and Plato—claimed that something actual always exists. For they said that motion had always existed even before the world. Leucippus attributed motion to the atoms, which are mobile of themselves, from which he supposed the world to be composed; and Plato attributed it to the elements, which he said were moved by disorderly motions before the formation of the world, and afterwards were brought into order by God.

2505. Now, they seem to be right in claiming that motion has always existed. But they were wrong in failing to point out which kind of motion has always existed; nor did they give the cause of motion, either by stating this in an absolute sense or by giving the reason for their own position. Yet nothing is moved by chance, that is, without some fixed cause, but there must always be something existing that is the cause of motion. For example, we now see that some things are moved in this way by nature or by force or by mind or by some other agent. Hence they should also have stated what the first cause of motion is, whether it was nature or force or mind. For it makes a great deal of difference which of these is held to be the cause of motion. But Plato cannot be excused on the ground that he held the principle of motion to be something that moves itself, which he asserted to be a soul, since the soul did not exist of itself before the formation of the world, but only existed after the disorderly state of motion. For, according to him, the soul was created at the same time as the heavens, which he claimed to be animated; thus it could not be the principle of that disorderly motion.

2506. Now, to think (1072a3). Then he answers the question which was raised, and concerning this, he does two things.

First, he returns to the points established in book 9 regarding the relationship of potency to act. He says that the opinion that potency is prior to act is in one sense right and in another not. The sense in which it is right has been explained in book 9 (1049b4–1049b17; [1844–49]), for it was stated there that act is prior absolutely to potency. But in one and the same subject that is being moved from potency to act, potency is prior to act in time, although act is prior both in nature and in perfection.

2507. That act is prior (1072a4). Second, he strengthens his answer by giving the opinions of some of the philosophers. He says that the absolute priority of act is asserted by Anaxagoras, because he claimed that the first principle of motion is an intellect, for intellect is a kind of act. The same thing is also asserted by Empedocles, who claimed that love and strife are the causes of motion, and also by Leucippus and Plato, who claimed that motion has always existed.

2508. Hence Chaos or Night (1072a7). Then he uses the answer to the question given above to clarify a point previously established, and in regard to this, he does three things.

First (1072a7; [2508]), in the light of the things established above, he concludes that generation must be eternal.

Second (1072a9; [2510]), on the ground that generation is eternal, he concludes that the motion of the heavens must be eternal, at therefore, if something.

Third (1072a23; [2517]), on the ground that the motion of the heavens is eternal, he concludes that the first unmoved mover must be eternal, at therefore, there is.

He accordingly first says (1072a7) that, if act is prior absolutely to potency, it follows that it is false to hold that all things preexisted potentially for an infinite time in a kind of confused mass, which was called Chaos by the ancient philosophers of nature, who thought potency to be prior absolutely to act. And the opinion of the theological poets is also false, who claimed for the same reason that the simple privation of things had existed for an infinite time before things began to be actually. Some called this privation of things Night, and perhaps the reason for their doing so is that, among qualities and simple forms, light is found to be more common and prior (since they thought that nothing exists except sensible things), and night is the privation of light. Both opinions are false, then, if act is prior to potency.

2509. But since we see that things that are generated and destroyed pass from potency to act, it will be necessary to say that the same things that begin to be actually after being potentially have always existed in some way, or according to circular generation, inasmuch as they claimed that things that are generated were formerly the same specifically but not numerically. And this is what occurs in circular generation, for from the moist earth, vapors are derived, and these turn into rain, by which the earth is again made moist.

Similarly, sperm comes from a man, and again a man is generated. Thus things that come to be are brought back the same in species by reason of circular generation. Or it is that those things that come to be actually after being potentially have always been the same things in a different way, according to Anaxagoras’ claim that they had actual prior existence in the things from which they are generated.

2510. Therefore, if something (1072a9). Then he concludes that the motion of the celestial bodies must be eternal on the ground that generation is eternal.

Therefore, granted that there is no other motion by which things that pass from potency to act have always been the same except that which proceeds according to the cycle of generation, he concludes from what has been shown in the philosophy of nature (especially in On Generation and Corruption 2) that, if something remains the same throughout the cycle of generation, something must also remain numerically the same, which will act in the same way so as to cause the eternal motion of things. For none of the things that are generated and destroyed can be the cause of the eternity that is found in generation and destruction, because no one of them always exists, nor even all of them, since they do not exist at the same time, as has been shown in Physics 8. It follows, then, that there must be some eternal agent that always acts in a uniform way so as to cause the eternal motion of things. This is the first heaven, which is moved and causes all things to be changed by its daily motion.

2511. But that which always acts in the same way only causes something that is always in the same state, and obviously those things that are generated and destroyed do not remain in the same state, for at one time they are generated and at another destroyed. This being so, if generation and destruction are to occur in the realm of lower bodies, it is necessary to posit some agent that is always in different states when it acts.

He says that this agent is the body that is moved in the oblique circle called the zodiac. For, since this circle falls away on either side of the equinoctial circle, the body that is moved circularly through the zodiac must be at one time nearer and at another farther away, and it causes contraries by reason of its being near or far away. For we see that those things that are generated when the sun comes closer to the earth are destroyed when the sun recedes; for example, plants are born in the spring and wither away in the autumn. For both the sun and the other planets are moved in the circle of the zodiac. But the fixed stars are also said to be moved over the poles of the zodiac and not over the equinoctial poles, as Ptolemy proved. And the coming to be and ceasing to be of everything that is generated and destroyed is caused by the motion of these stars, but more evidently by the motion of the sun.

2512. Therefore, this mover which acts in different ways must act in one way of itself, that is, by its own power, inasmuch as it causes the diversity found in generation and destruction. And it must act in another way in virtue of something else, that is, by the power of some other agent, inasmuch as it causes eternal generation and destruction. Hence, this second agent must act either in virtue of some third agent, that is, by the power of some other agent, or of the first, that is, by the power of the first agent, which always acts in the same way.

And since it is not possible to assign some other agent by whose power this first agent brings about the eternal motion of things, it is therefore necessary according to this that it act in the same way; namely, that, by its power, it causes the eternal generation and destruction of things. For the first agent, which always acts in the same way, is the cause of that which acts in different ways. For that which acts in different ways acts eternally, and that which acts in the same way is the cause of the eternity of any motion. Hence it is the cause of the eternity of that which acts in different ways inasmuch as the latter acts eternally in this way; and it is also the cause of that which is produced by it, namely, eternal generation and destruction. From this it is also evident that the second agent, which acts in different ways, acts by the power of the first agent, that is, the first heaven or first orb, which always acts in the same way.

2513. Hence it is clear that the first agent, which always acts in the same way, is more powerful and more noble, because it is the cause of that whose being is always to be the same, that is, of eternity. But the cause of that whose being is to be different is another agent, which acts in different ways. And it is evident that both of these combined—that is, both the first agent, which always acts in the same way, and the second agent, which acts in different ways—are the cause of that which both always is and is in different states, namely, the fact that generation and destruction are eternal.

2514. Again, he concludes from this that, if the motions of the heavens are such that eternal generation and destruction in the realm of lower bodies can be caused by them, it is not necessary to look for any other principles (such as the ideas, which the Platonists posited, or love and hate, which Empedocles posited), because it is possible to account for the eternal generation and destruction of things in the above way.

2515. And if this way is not accepted, the untenable conclusions to which the first philosophers were led will follow—namely, that all things will come from Night, that is, from a simple privation, or that all things were together, or that something comes from non-being.

2516. Therefore, it is evident that, if the above-mentioned position is accepted—that eternal generation and destruction are caused by the eternal motion of the heavens—the foregoing untenable conclusions are eliminated. And it will follow that something is always being moved in an unceasing motion, which is circular motion. This becomes apparent not only by reasoning but from the effect itself and by perception. Hence, since the first heaven always causes motion by means of this motion, it must be eternal.

2517. Therefore, there is (1072a23). From what has been said above, he next infers that there is an eternal unmoved mover. For, since everything that is being moved is being moved by something else, as has been proved in the Physics, if both the heavens and their motion are eternal, there must be an eternal mover. But since three kinds are found among movers and things moved—the lowest of which is something that is merely moved, the highest something that moves but is unmoved, and the intermediate something that both moves and is moved—we must assume that there is an eternal mover that is unmoved. For it has been proved in Physics 8 that, since there cannot be an infinite number of movers and things moved, we must come to some first unmoved mover. For even if one might come to something that moves itself, it would again be necessary to come to some unmoved mover for the above reason, as has been proved in that work.

2518. Again, if the first mover is eternal and unmoved, it must not be a potential being (because any potential being is naturally fitted to be moved), but an independent substance whose essence is act. This is the conclusion which he drew above (1071b20; [2499]). But it was necessary to raise this question, which was discussed among the ancients, so that, when it has been solved, the course to be followed in reaching a primary being whose substance is act will be made more evident.