Lecture 10 — On the nature of the stars

289 We have next to speak of the stars, as they are called, of their composition, shape, and movements.

290 It would be most natural and consequent upon what has been said that each of the stars should be composed of that substance in which their path lies, since, as we said, there is an element whose natural movement is circular.

291 In so saying we are only following the same line of thought as those who say that the stars are fiery because they believe the upper body to be fire, the presumption being that a thing is composed of the same stuff as that in which it is situated.

292 The warmth and light which proceed from them are caused by the friction set up in the air by their motion. Movement tends to create fire in wood, stone, and iron; and with even more reason should it have that effect on air, a substance which is closer to fire than these. An example is that of missiles, which as they move are themselves fired so strongly that leaden balls are melted; and if they are fired the surrounding air must be similarly affected.

293 Now while the missiles are heated by reason of their motion in air, which is turned into fire by the agitation produced by their movement, the upper bodies are carried on a moving sphere, so that, though they are not themselves fired, yet the air underneath the sphere of the revolving body is necessarily heated by its motion,

294 and particularly in that part where the sun is attached to it. Hence warmth increases as the sun gets nearer or higher or overhead.

295 Of the fact, then, that the stars are neither fiery nor move in fire, enough has been said.

382. After settling the question of the heaven, the Philosopher now treats of the stars.

First, he determines the truth;

secondly, he raises and settles some doubts, at there are two difficulties (L. 17; 291b24).

As to the first he does four things:

first he determines the nature of the stars;

secondly, their motion, at since changes evidently occur (L. 11; 289b1);

thirdly, their order, at with their order (L. 15; 291a29);

fourthly, their shape, at with regard to the shape of each (L. 16; 291b11).

With respect to the first he does three things:

first, he states his intention;

secondly, he manifests the truth, at it would be most natural (289a13; [383]);

thirdly, he excludes an objection, at the warmth and light (289a19; [387]).

He says therefore first (289a11) that, after having determined the question of the whole heaven, we should next consider those things called "stars" and determine what they are made out of, i.e., of what nature they are, and what shape they have, and of what sort are their motions.

383. Then, at it would be most natural (289a13), he shows of what nature the stars are.

First he proposes what he intends, and says that the assumption that each star is of the nature of the spherical body in which it is moved is of itself most reasonable, since places follow upon the natures of bodies. Hence it is reasonable that the stars pertain to the nature of the sphere in which they are situated. Moreover, this follows of necessity from what we have said above. For it was said that the heaven has a nature other than the nature of the four elements, since it has a motion distinct from the motions of the [four] elements, namely, a circular motion. Hence, since the stars are moved circularly as the heavenly spheres are, the consequence is that they have a nature in common with the heavenly spheres and differ from the nature of the four elements.

384. But there seems to be a twofold doubt on this matter.

The first one arises from the fact that the bodies of the stars appear to differ from the bodies of the heavenly spheres, since they shine and seem to be more compact; consequently, there seems to be contrariety in the heavenly bodies. Now contrariety is the cause of corruption. It seems, therefore, that heavenly bodies are according to their nature corruptible, which is against what has been determined in Book I.

The answer to this is that not every diversity, strictly speaking, has the notion of contrariety. Rather, in order that things be contrary, two things are required: one of these is that they be apt to exist in some way in the same subject, whether proximate, or at least remote—thus heat is contrary to cold, which latter, however, is not apt to exist in fire, but is apt to exist in the matter of fire, which is the first subject. Secondly, it is required that the diverse things that are contrary not be able to exist together, but mutually expel one another. Hence, white and black, as existing in matter, are contraries mutually expelling one another. But as existing in the intellect they have no contrariety but can exist together; as a matter of fact, one of them is known through the other.

Now the differing forms or qualities that seem to be in the heavenly bodies are in no sense apt to exist in the same subject, whether it be a proximate subject or a first subject — for the body of a star is not apt to be reduced to the disposition of the other parts of the heavenly spheres, or vice versa. Similarly, it is also necessary to say that the contrary forms or qualities found in the lower bodies exist in some sense in heavenly bodies, not, indeed, in a univocal way, but as in universal causes, in a certain likeness to the manner in which forms that exist individually in sensible matter, exist universally in the intellect. Hence, just as in the intellect they do not exist under the notion of contrariety, so neither do they in the heavenly bodies. Hence Plato said that in the heavenly bodies are found the excellences or sublimations of the elements, the former being, as it were, their primordial active principles — for the heavenly bodies are related to elementary bodies as are active principles to passive. Therefore, the opposite happens in heavenly bodies and in elementary bodies. For elementary bodies, the more compact they are made as a result of thickening, the more material, and passive, and less light-endowed they are. This is evident with respect to earth, which is the dominant factor in mixed bodies. On the other hand, in the heavenly bodies, the more compact they are as a result of thickening, the greater their luminosity and active power, as is plain in the bodies of the stars.

Consequently, it is plain that such diversity as is apparent in the heavenly bodies is not seen to possess the nature of contrariety. Therefore, it does not follow that they are susceptible to corruption. But that would follow if there were true contrariety, as Aristotle showed in Book I.

385. The second doubt is to the effect that, since diversity appears between the stars and the remaining parts of the spheres, it seems that they are not simple bodies. But the answer to this is that these bodies are called simple to the extent that they are not composed of contrary natures. Yet there is in them some diversity with respect to the nature of their species, although they agree in the nature of their genus—thus they agree in the common aspect of their motion, because all are moved circularly.

386. Secondly, at in so saying we are only following (289a16) he shows that this is in a way in agreement with the statements of others. For he says that, just as those who assert that the stars are fiery, concluded this because they believe the heavenly body is fire, as though it were reasonable for each of the stars to consist of the nature of those spheres in which it exists; so, too, we assert that the stars are of a nature different from the nature of the four elements, because we have previously proved the heavens to be such.

387. Then, at the warmth and light (289a19) he excludes an objection; for some believed the stars to be of the nature of fire and argued in the following manner: To be hot and luminous is seen to be proper to fire. But the stars heat and give light. Therefore, it seems that they are of the nature of fire. Regarding this he does three things:

first he resolves this objection with an example;

secondly, he shows how the example differs from his proposition, at now while the missiles (289a26; [388]);

thirdly, he replies to a tacit question, at and particularly in that part (289a31; [389]).

He says, therefore, first, (289a19) that heat and light are generated by the stars by a certain stroking of, or friction with, air and not because they are of a fiery nature. For we observe that motion is apt to ignite wood and stones and iron; hence it is all the more reasonable that a body which is more like fire than the bodies mentioned should be ignited by motion, because when two things are more alike, it is easier for them to change into each other. But air is more like fire than the above mentioned bodies are. Hence air can be more easily ignited by motion than those bodies. And he gives the example of arrows. Part of them being of lead, they are so heated by the violence of their motion that sometimes the lead melts. Since arrows can be ignited as a result of motion, there is all the more reason why the air surrounding the arrows should become ignited. This does not mean that the heated arrow causes the air to become heated, as Simplicius understood. Rather, it is to be understood that the heat produced in the air through motion causes the heating of the arrows, as Alexander explained it. For Aristotle wishes to prove from a lesser case that if arrows are heated, the surrounding air must be heated, it being closer to fire, as he said above. He is not taking the case [of the heated arrow] as a cause, as Simplicius understood.

388. Then, at now while the missiles (289a26) he shows the difference between this example and his proposition. And he says that those arrows become hot because they are being moved through air, and the air becomes ignited by the blow, i.e., as a result of its being struck and divided by the arrow; hence it is from contact with heated air that the arrows become hot. But this is not what happens in the stars, since each star is not being moved through air but in its own sphere insulated from air. Therefore, the stars are neither ignited nor made hot, first of all because they are far from air, which is ignited by motion, and, secondly, because they are not receptive to a transitory impression. But it is the air which exists under the sphere of the heavenly body that must become hot as a result of the motion of the heavenly sphere, since the motion of the heavenly sphere agitates not only the fire but the air as well (namely, as far as the air contained below the mountains), as is apparent from the motion of comets, as stated in Meteorology I.

389. Then, at and particularly in that part (289a31) he responds to a tacit objection. For if the lower air is ignited by the motion of the heavenly sphere, then, since the heavenly sphere is in continuous motion, it seems that there should always be an equal amount of heat in the air at all times, i.e., in winter and in summer, during the day and during the night. But we observe that it is the contrary that happens. To this he responds that air is mainly ignited by the motion of the particular sphere in which the sun is fixed. Therefore heat is generated on account of the sun's nearness to us. And this happens in two ways: first, insofar as by its rising, the sun ascends to our upper hemisphere; secondly, insofar as it reaches a point directly overhead—for just as there is more heat during day than during night, so too there is more at noon than at dawn.

Lastly, he concludes (289a34) by way of summary that so far we have said that the stars are not fiery in nature, and are not moved in a fiery body, but above the sphere of fire, in the heavenly spheres.

390. But a first doubt arises: since Aristotle asserts that the motion of the stars generates heat and light, his proof seems insufficient, since he gives no evidence for light, but only for heat.

To this Alexander responds that he reserves his treatment of light for settling in his book, On the Soul, in the second part of which he states that light is not a peculiarity of fire but something common to it and to the supreme body. However, since Aristotle says that both are generated from the striking of the air, it is better to say that Aristotle has taken care of both right here by showing that as a result of the stars' motion the lower bodies are set afire—for in fire are found both heat and light.

391. But a doubt still remains, namely, the doubt as to that nature whence motion has the power to ignite or heat things.

To this Averroes responds in his Commentary that it is proper to the hot to be mobile; consequently, when something is being moved in act, it also becomes hot in act. But this is not seen to be true.

First of all, because to be moved is not peculiar to the hot but is common to every natural body—for things moved with a straight motion rest when they are in their appropriate places, but are moved when they are outside their places; heavenly bodies are moved circularly in their places, which are neither hot nor cold.

In the second place, the subsequent is not the cause of the prior—hence, if motion were proper to what is hot, heat would be the cause of motion rather than motion the cause of heat.

Therefore, it must be said that, as is proved in Physics VIII, local motion is the first of motions. Now in every genus, whatever is first is the cause of the things that are subsequent in that genus. Hence, local motion is the cause of alteration, which is the first motion among the other motions, and in particular the cause of the first alteration which is heating. For alteration in all the other qualities is caused by alteration in the first four qualities, among which the two active, namely, hot and cold, are prior to the passive, namely, moist and dry. But hot is prior to cold, as form is prior to privation, as is clear from what was said previously. Hence, local motion is properly the cause of heat. However, every local motion possesses this from the power of the heavenly motion, which is the first of local motions.

392. Another doubt now arises: Since the sun immediately touches neither the air nor the fire, how is heat caused in the air and in fire from the motion of the sun? For the intermediate heavenly bodies, namely, the spheres of Venus, Mercury and the moon, are not heated from the motion of the sun.

To this Alexander responds that there is nothing preventing a thing from being altered by an agent acting through a medium, without the medium itself being altered. For example, the fish called stupor shocks the fisherman's hand through the net, although the net does not shock. Yet the net does receive in some way the impression from the fish, but in a manner different from the way the hand does. So, too, the sun imprints something on the intermediate heavenly bodies, even though it is not heating; but the effect of the sun reaches the lower bodies after the manner of heat, in keeping with their condition. But against this answer appears to be the fact that Aristotle asserts that heat is caused in air that has been struck or compressed by the motion of the stars. But it is not possible for this striking or compressing effect from the motion of the sun to reach the air without the intermediate heavenly bodies also being struck, which is impossible.

Therefore Averroes in his Commentary says that the entire heavenly body is moved with the diurnal motion as though it were one body, or one whole animal, whereas the particular motions of the planets are as motions of the parts of an animal. Now heat in the air is caused mainly by the motion of the whole heaven, i.e., by the diurnal motion. For this reason Aristotle says that heat is caused by the nearness of the sun when it rises and is overhead, all of which is due to the diurnal motion. Now, it is plain that when one body alters another it does not do so only according to its outermost surface where it touches the body altered, but does this according to its whole depth or mass—and a sign of this is that a thin body is not as well suited for causing alteration as a body having thickness or mass, supposing that both bodies possess the same nature. Thus the whole heaven causes heat, not only in virtue of the lowest sphere, but in virtue of the whole mass of the heaven, as though by a single alteration. Therefore, alteration takes place in those lower bodies not only in virtue of the moon's orb, which is in immediate contact with the lower bodies, but also according to the virtue of the stars, in which the active power of the heavenly body is more unified and as though massed together. This is particularly true of the sun, which exceeds the other bodies in power and size. Since, therefore, the whole heaven acts as one body according to its diurnal motion, we should not suppose that one sphere acts separately upon another, but that the whole heaven, with one imprint, alters the lower air by means of the power in the sun and other stars as they draw near to us.

But, as Simplicius says, even this explanation does not seem to be sufficient.

First, because if heat were produced in the air on account of its being struck or compressed by the motion of the heavenly body when the sun is present, it would follow first of all that lower places would become less heated, since they are farther from the heating motion. But just the opposite is seen, for there is more heat on the plains than on the mountains.

Secondly, since the sphere of the earth is almost like a point in relation to the sphere of the sun, when the sun is over the earth it seems to be more or less equally near to every part of the earth; consequently, there should not be such a great variation of heat from the sun as there is between dawn and noon, or winter and summer.

Thirdly, because there would be no reason for finding less heat in shaded places than in places where the rays of the sun strike. The same reasons prove that the sun does not cause heat as though it were of the nature of fire.

And therefore Simplicius says that there emerge from the body of the sun rays he says to be corporeal, and that they can without hindrance penetrate those heavenly bodies below the sun that are immaterial, and air they penetrate on account of its pores. But they are reflected by solid bodies, namely, earth and water, at equal angles (because, as perspective proves, all reflection occurs at equal angles). When, therefore, a solar ray strikes the earth diametrically, it is reflected back upon itself and thus there is produced a maximum thickening, which causes maximum heat; and this happens when the sun is directly overhead. But as the sun declines from its overhead position, the distance between the reflection of its rays increases more and more, and consequently less heat is generated. This is the reason why in the winter, and at sunrise and sunset, there is less heat in the air, namely, because the solar rays strike the earth at angles which are more obtuse, with the result that the distance between the rays first striking and the reflected rays is increased. And it is for this reason that Aristotle purposely did not say unqualifiedly that as the sun gets nearer more heat is generated, but was careful to add, as rising and when it is overhead, in order to give us to understand that the nearness is in reference to its nearness overhead and not in reference to the linear distance from the sun to us, because those distances to any part of the earth are almost equal.

Now if Simplicius means with these words that the rays are bodies which strike and thicken the air and thus produce heat, his statement is expressly false, for Aristotle in On the Soul II proves that a ray is neither a body nor an emanation from a body. But if, in saying that the rays are corporeal, he means that they act after the manner of bodies, in the sense that they are directly projected and are reflected from any dense body they cannot penetrate, then he speaks the truth—for such reflections, being caused by the counter-resistance of bodies, belong not only to bodies but also to qualities, since heat, too, is reflected when it meets an obstacle, as do other things of the same sort.

393. However, if one examines them diligently, all the things said on this point are true in some sense. For Aristotle says that heat and light are generated by the stars as a result of the air being struck by their motion. Now it seems that this is not to be understood as meaning that heat and light are generated by the striking of the air caused by the motion of the heavenly bodies: for it is not a question here of the light of fire generated from motion, as previously stated, but of the light caused by the stars themselves insofar as they are light-giving things in act. Therefore, the cause of heat generated in lower bodies by the heavenly bodies is twofold: One cause is motion, and the other is light. Why motion is the cause of heating has been explained above. But we must not suppose that a mutual stroking or rubbing of a heavenly body and of air is the cause of heat; rather it is solely the motion produced in the air by the higher motion of the heavenly body. The higher air, and likewise fire, are moved by the diurnal motion of the entire heaven according to the power of the sun and all the stars, as Averroes says. The second cause of the heating of lower bodies by the stars and especially the sun is light. It indeed has the power to heat insofar as it is an active quality of the first thing altering, namely, the heaven. Hence, it directly causes the first quality found in inferior bodies, namely, heat. And since this quality, namely, light, abounds more in the sun, the sun is especially capable of heating. But the rest of the heavenly bodies, insofar as they partake of light (which is the universal active power of heavenly bodies), have the power to produce heat. This extends even to the light of the moon, according to what the Philosopher says in his book, On the Parts of Animals, namely, that the nights of full moon are warmer, for which reason certain fish come to the surface of the water.

As to the statement that some stars cause things to be cool or moist, Averroes says in his Commentary that they do not do this per se but only insofar as producing a heat proportionate to each body; on which account he scolds Avicenna, who says that the stars cause both cooling and heating. But Averroes does not speak correctly in this matter. For a thing is seen to be per accidens when it is not produced by the agent per se. But the heavenly bodies are the agent causes of the things that are here. If, therefore, they did not per se cause cooling and moisture and so on, it would follow that such things would be per accidens in the universe. Moreover, since all the substantial forms of the lower bodies come to be from the power of the heavenly bodies, it follows that from their power are also the qualities consequent upon the species or forms of the elements, namely, hot, cold, wet and dry, and so on.

Accordingly, it must be said that all the heavenly bodies, in keeping with their common power of light, have the property of heating, but according to other peculiar powers attributed to specific bodies, they not only heat and cool, but also produce other corporeal effects in lower bodies. And in the matter of the influence of light and these powers, what Alexander says is true, namely, that the intermediate heavenly bodies receive an impression from the sun in a way different from the way the lower bodies do.

One should consider, therefore, that accordingly as heat is caused in lower bodies by the motion of the stars and of the whole heaven, the bodies nearer to the heaven, namely, fire and the upper region of the air, which are carried around according to the motion of the heaven, are hotter. But accordingly as heat is caused from the light of the stars, the lowest things are hotter, because in the upper regions the reflections of the rays are dispersed more. This also explains why many species of things on the earth are generated by the power of solar and stellar rays which are multiplied about the earth as a result of reflexion.

394. But Alexander raises a question here: If the heavenly bodies strike the air in their motion, it seems to follow that they can be touched and, consequently, that they are hot and cold—for those are the first tangible qualities, as is said in On Generation II.

But the answer to this is easily evident from what Aristotle says in On Generation I, namely, that things apt to act and be acted upon touch each other mutually, and that the qualities of such things are heat and cold. The heavenly bodies, however, act but are not acted upon. Hence, they touch without being touched. Hence, too, the tangible qualities in heavenly bodies are not there in the way they are in lower bodies; rather, they are present in a more eminent way, as existing in their active cause. For hot and cold, moist and dry, are not present therein, but the power that causes them is present therein. Similarly, neither do heavy and light exist there; but in place of these is an aptitude to circular motion. Finally, rare and dense are present in heavenly bodies, accordingly as the stars are thicker and more massive than their spheres; nevertheless this is not according to the difference of contrariety, but only according to increase or decrease of power proportionate to a greater or lesser aggregation of parts.