Lecture 4 — The reason why there are in the heaven, several spheres moved with a circular motion

248 Since circular motion is not the contrary of the reverse circular motion, we must consider why there is more than one motion,

249 though we have to pursue our inquiries at a distance—a distance created not so much by our spatial position as by the fact that our senses enable us to perceive very few of the attributes of the heavenly bodies. But let not that deter us.

250 The reason must be sought in the following facts. Everything which has a function exists for its function. The activity of God is immortality, i.e. eternal life. Therefore the movement of that which is divine must be eternal. But such is the heaven, viz. a divine body, and for that reason to it is given the circular body whose nature it is to move always in a circle.

251 Why, then, is not the whole body of the heaven of the same character as that part? Because there must be something at rest at the center of the revolving body; and of that body no part can be at rest, either elsewhere or at the center. It could do so only if the body's natural movement were towards the center. But the circular movement is natural, since otherwise it could not be eternal: for nothing unnatural is eternal. The unnatural is subsequent to the natural, being a derangement of the natural which occurs in the course of its generation. Earth then has to exist; for it is earth which is at rest at the center. (At present we may take this for granted: it shall be explained later.)

252 But if earth must exist, so must fire.

253 For, if one of a pair of contraries naturally exists, the other, if it is really contrary, exists also naturally. In some form it must be present, since the matter of contraries is the same.

254 Also, the positive is prior to its privation (warm, for instance, to cold), and rest and heaviness stand for the privation of lightness and movement.

255 But further, if fire and earth exist, the intermediate bodies must exist also: each element stands in a contrary relation to every other. (This, again, we will here take for granted and try later to explain.)

256 [Since this is so, in] these four elements generation clearly is involved, since none of them can be eternal: for contraries interact with one another and destroy one another.

257 Further, it is inconceivable that a movable body should be eternal, if its movement cannot be regarded as naturally eternal: and these bodies we know to possess movement. Thus we see that generation is necessarily involved.

258 But if so, there must be at least one other circular motion: for a single movement of the whole heaven would necessitate an identical relation of the elements of bodies to one another. This matter also shall be cleared up in what follows:

259 But for the present so much is clear, that the reason why there is more than one circular body is the necessity of generation, which follows on the presence of fire, which, with that of the other bodies, follows on that of earth; and earth is required because eternal movement in one body necessitates eternal rest in another.

330. After discussing the diversity of positional parts of the heaven, the Philosopher here settles the question of the diversity of parts based on the order of the spheres, and explains why there is not just one circularly-moved sphere in the heaven but several of them. Concerning this he does three things:

first he states the problem;

secondly, he explains why this problem is difficult to solve, at though we have to pursue (286a4; [332]);

thirdly, he begins to solve it, at the reason must be sought (286a7; [333]).

331. In regard to the first it should be kept in mind that if circular motions were contrary, it would not be difficult to see why there are many circular motions in the heaven and not just one. For since contraries differ specifically, inasmuch as contrariety is a difference with respect to form, as is said in Metaphysics X, the universe would not be perfect in its species if there were one contrary motion and not another, for example, if there were downward motion and not upward motion. Since, therefore, it has been previously proved that one circular motion is not contrary to another, we should inquire diligently as to the necessity of having many and diverse circular motions in the heaven. This question quite logically follows our previous discussion where it was said that up and down, and other such are assigned to the heaven in one way with respect to the first motion, and in another way with respect to the second.

332. Then, at though we have to pursue (286a4), he points out the difficulty of solving this question. For that is difficult for men to consider when the question is from afar, i.e., concerning the heavenly bodies far removed from us, since concerning things distant from us we cannot have a certain judgment. However, heavenly bodies are not only removed by so much, i.e., according to the quantity of local distance, but much more by the fact that few of their accidents fall on our sense observation, whereas it is connatural for us to pass from the knowledge of accidents, i.e., sensible things, to an understanding of a thing's nature. He states this distance to be much more than the local distance. For if we should consider the local distance, there is some proportion between the distance separating us from a heavenly body and that separating us from various lower bodies, e.g., a stone or some wood, and both distances are in one genus, but the accidents of heavenly bodies have a different notion and are wholly improportionate to the accidents of lower bodies. Yet, in spite of the difficulty, let us inquire into the "why" of the variety of heavenly motions. And the cause of this variety will be derived from what we are about to say.

333. Then, at the reason must be sought (286a7), he assigns the cause of the aforesaid.

First he assigns it by the method of composition [synthesis], going from what is first to what is last in the order of inquiry;

secondly, by that of resolution [analysis], proceeding from what is last sought to what is first, at but for the present (286b6; [343]).

With respect to the first (286a7) he gives the following argument: If the heaven is a certain divine body, its motion must be eternal and circular; if its motion is eternal and circular, earth must exist; if earth exists, fire must exist; if fire and earth exist, there must be intermediate bodies; if such bodies exist, there must be generation; if there is generation there must be several motions in the heaven. Therefore, if the heaven is a perpetual and divine body, there must be a number of motions in the heaven, and consequently a number of mobile bodies.

334. Then he explains each of these conditionals in order: and first of all the first one. With respect to this it must be remembered that the Platonists posited one supreme God Who is the very essence of goodness and unity, under Whom they put a superior order of separated intellects, which we are accustomed to call "intelligences"; under this order they placed the order of souls, and under this the order of bodies. They said, therefore, that among the separated intellects the first and superior ones are called "divine" intellects on account of their likeness and nearness to God; but the others are not 'divine, because of their distance from God. Similarly, among souls the supreme are intellective, while the lowest are not intellectual but irrational. The supreme and nobler bodies were said to be animate; the others inanimate. Furthermore, they said that the supreme souls, because they depend on the divine intelligences, were divine souls; and the supreme bodies, because united to divine souls, divine bodies.

Now, Aristotle uses this manner of speaking here. He says that anything which possesses its on operation exists for its operation, for everything seeks its perfection as its end; but operation is the ultimate perfection of a thing (or at least the product of the operation is, in the case of those things in which there is some product beyond the operation, as is said in Ethics I). For it is said in On the Soul II that form is first act, and operation second act as the perfection and end of the thing acting. And this is true both in material and in spiritual things (such as habits existing in the soul), and in natural as well as artificial things. And he adds the expression, "which produce a work" to take care of things contrary to nature, such as monstrosities, which produce no work as such, but suffer a defect in their operational power, as is plain in those born lame or blind. For lameness is not an end intended by nature, for the sake of which it makes an animal to be born lame; such a thing happens outside the intent of nature, by reason of a defect in the natural principles.

He adds further that God's operation is immortality, where "God" refers not only to the first cause of all things but to all things called "divine" according to the custom of the Platonists and other gentiles. But immortality appears not to be an operation but rather a difference or an impassibility. To this it must be answered that immortality designates unfailing life. Now life refers not only to the existence of the living being, but to its operation as well, as understanding is a certain living and as are sensing and other such, as is plain from On the Soul II and Ethics IX. This point is brought out when he says further that this, namely, immortality, is eternal life—for which reason he does not say that God's operation is indestructibility, which refers only to the eternity of His existence but he says it is immortality, in order to include eternity of action. Hence he concludes that if any mobile being is called "God" in this sense, its motion is eternal; just as, if some immobile substance is called "God," its operation is eternal without motion. If this were not so, then such a thing, eternal in existence but lacking an eternal operation (since things exist in order to act) would exist without a purpose.

Therefore, since the heaven is such as to have been called "God" by the ancients, not indeed because it is the supreme God, but because its body is something divine by virtue of being ungenerated and indestructible, as was previously explained; consequently it possesses a circular body in order that it may be moved forever and in a circular way. For it has been shown in Physics VIII that only circular motion can be eternal, since there is no motion over an infinite straight line, as was also proved in Book I, and motion over a finite straight line cannot be infinite except by reflexion, which involves interposing states of rest, as was proved in Physics VIII.

It should be noted that Aristotle here proves the eternity of the motion of the heaven from the eternity of its body; but in Physics VIII he did not use this way, because he had not yet proved the eternity of the heaven. But because the heavenly body is related to the motion of the heaven as its matter and subject, whereas the first mover, namely, God, is the agent making it actually exist, then from the side of the heaven it can be preyed able to be moved forever, but on the part of the divine will depends whether it is to be actually moved forever or not forever.

335. Then, at why, then, is not the whole body (286a12) he proves the second conditional, namely, that if the heaven is moved with an eternal and circular motion, earth must exist. He says, therefore: if it is true that the heaven is a divine body which is eternally and circularly moved, why then does not the entire heaven, i.e., the entire world, have such a body, i.e., why does not each part of the world have the nature of heavenly body?

To this he answers that there has to be something permanent and at rest in the middle [center] of the body which is in circular motion—for it is plain that every circular motion goes around something at rest. And this something must be a body: for the thing I am calling the middle [center] is not something subsistent, but an accident of a bodily thing, if it is the center of a body. Now such a thing cannot be a part of this, i.e., of the heavenly body, which has already been described as divine, although it must be a part of the whole world. This he proves in two ways.

First, because no part of a heavenly body generally can be at rest everywhere, since to the heavenly body belongs eternal motion, as was shown. But the center about which there is circular motion must be stationary.

Secondly, because taken specifically it cannot rest in the center. For if it rested there naturally, it would be naturally moved to the center (for each thing is naturally moved to the place in which it rests naturally, as was explained in Book I). But no part of the heavenly body is naturally moved to the center, because its natural motion is for it to be moved circularly, and, as was explained in Book I, no simple body can possess two natural motions.

Hence it remains that the rest of a part of the heavenly body in the center would be against its nature. From this it follows that the motion of the heaven could not be eternal: because it cannot take place unless there is something at rest in the center, and if the state of rest of that of it which was in the center were violent, it would follow that it [the rest] could not be eternal, and consequently neither could the heaven's motion be eternal. For nothing contrary to nature is eternal—since what is contrary to nature is subsequent to what is according to nature. This is plain from the fact that in the generation of anything, whatever is outside nature is a kind of excess, i.e., a corruption and defect of that which is according to nature (for example, monstrosities are certain corruptions and defects of a natural thing). But corruptions and defect are naturally posterior, just as privation is subsequent to possession. Now it is not possible that something naturally prior should never exist, and that what is naturally apt to be later, should always exist. Consequently, it is plain that what is violent cannot be eternal. But that which is at rest in the center, is eternally at rest, just as the heaven is eternally in motion. What is left, therefore, is that there must be something naturally at rest in the center, if the motion of the heaven is circular and eternal. But it is the earth that is naturally at rest in the center, as will be proved later. Therefore, if the heaven is moved circularly and eternally, then earth must exist. And this is what was proposed to be proved.

336. Then, at but if earth must exist (286a22) he proves the third conditional, namely, if there is earth, there is fire.

First he proposes what he intends, and says that it is also true that if earth must exist, then fire must exist.

Secondly, for, if one of a pair of contraries (286a23), he proves this with two arguments.

The first is this. If one contrary exists in nature, the other also must exist in nature. He proves this as follows: If there be any contrary, there must be a matter subject to it, as is plain from Physics I; but the matter of contraries is the same, as is shown in the same place. Consequently, the matter of one contrary must be in potency to the other. Now this potency would be in vain if that other contrary could not exist in nature. But since nothing is in vain in nature, then, if one contrary exists, so must the other. Now fire and earth are contraries, for they are the maximum distance apart, so far as contrariety of position is concerned, inasmuch as one is the heaviest of all things and the other the lightest (although in respect of other qualities, fire is especially contrary to water, as the hottest to the coldest—however, we are now speaking of bodies from the viewpoint of position, from which aspect they are parts of the whole universe). Consequently, if there is earth, there must also be fire.

337. The second argument is at, also, the positive is prior to its privation (286a25). With respect to it, it should be considered that contraries are always related according to "worse" and "better," as is said in Physics I, in such a way, namely, that one of them is a privation and a defect in comparison to the other, as are, for example, cold in relation to hot, and black in relation to white. Now, it is evident that affirmation, i.e., whatever is said positively as something complete, is prior to what is described in terms of privation and defect, as hot is prior to cold. But rest and heaviness, which are attributed to earth, are stated in terms of the privation of lightness and of motion, which are attributed to fire. Therefore, fire is naturally prior to earth. For if the subsequent is posited, so must be the prior. Consequently, if earth exists, fire too must exist. One should also note that Plato in the Timaeus proved that earth and fire exist on the ground that bodies have to be visible by virtue of fire and palpable by virtue of earth.

338. Then, at but further, if (286a28), he proves the fourth conditional, namely, that if fire and earth exist, so do the intermediate elements. For each of the elements is in some respect contrary to each of the other three, as earth is contrary to fire on the basis of heavy and light and cold and hot, and is contrary to air according to a contrariety of hot and cold as well as of wet and dry. And he says that this is to be made clear below, especially in On Generation II. Hence, it remains that if two of the elements exist, the other two must also exist, it having been proved that if one contrary exists, the other must.

Plato, however, used proportional numbers to prove that if there are extreme elements there must be intermediate elements. For between two numbers that are cubes there must be two other numbers according to a continuous proportion: for example, the cube of 2 is 8 and the cube of 3 is 27 and between these cubes the numbers 18 and 12 fall as proportional means, so that all can be arranged to form a ratio of 3 to 2.

339. Then, at since this is so, (286a31) he proves the fifth conditional, namely, that if such bodies exist, there must be generation and corruption. This he proves with two arguments.

The first of these is that contraries act upon one another and are acted upon by one another, and corrupt each other, as will be proved in the book On Generation. But the aforesaid bodies are contrary to one another, as has been said. Therefore they mutually corrupt one another. Consequently, generation and corruption must exist.

340. The second argument is at further, it is inconceivable (286a34) and is as follows: It is unreasonable to assume that there exists an eternal body whose motion cannot be eternal, because motion is the operation of a mobile body and each thing is for the sake of its operation, as has been said. But the above-mentioned bodies, namely, the elements, have straight motions which cannot be eternal, as was proved in Physics VIII. Therefore they can not be eternal but have to be generable and corruptible. Consequently, it is necessary that generation and corruption exist.

341. Then, at but if so (286b2) he proves the sixth conditional, namely, that, if there is generation, there has to be besides the first circular motion some other circular motion, either one or several. For since the first circular motion (which is that of the supreme sphere revolving the whole heaven from east to west) is uniform, it would not cause a diversity of dispositions in lower bodies. Consequently, the elements of bodies and other bodies would always retain the same disposition toward each other and there would be no generation and corruption. This will be explained more in detail later, namely, in On Generation II. Hence there must exist another motion, through the oblique [i.e., zodiacal] circle, to act as the proper cause of generation and corruption through the distance of the planets from us or their nearness to us, just as the first motion causes permanence and eternity in things.

342. Here Alexander asks if, supposing the motion of the heaven to cease, but the contrary elements to remain, they would corrupt one another? And, on the ground of their contrariety, he answers affirmatively; but not as though there would be reciprocal generation and corruption as now occurs when cold things are generated from hot and vice versa. That would happen would be what was posited by Heraclitus who said that at some time everything would become fire. For the present orderly mutual interplay of generation and corruption is traceable to the virtue of the heavens. But it is better to say that if the motion of the heavens were to cease, so too would the motion of all lower bodies, as Simplicius said. For the powers of the lower bodies are as matter and instruments in relation to the heavenly powers, and hence do not move unless moved.

343. Then, at but for the present (286b6) he summarizes the same argument in the resolutary [i.e., analytic] order. And he says that the reason why there must be several circularly moved bodies is now plain, namely, because there must be generation. But generation must exist, if fire and other bodies exist; these must exist" if earth exists; earth must exist, because there has to be something eternally at rest in the center, if something undergoing circular motion exists.