Lecture 4 — Three questions: the first, on the order of the elements, is solved

It remains to explain, after a preliminary discussion of difficulties, the relation of the two elements air and fire to the position of the first element,

and the reason why the stars in the upper region impart heat to the earth and its neighborhood.

Let us first treat of the air, as we proposed, and then go on to these questions. Since water is generated from air, and air from water, why are clouds not formed in the upper air? They ought to form there the more, the further from the earth and the colder that region is. For it is neither appreciably near to the heat of the stars, nor to the rays relected from the earth. It is these that dissolve any formation by their heat and so prevent clouds from forming near the earth. For clouds gather at the point where the reflected rays disperse in the infinity of space and are lost. To explain this we must suppose either that it is not all air which water is generated, or, if it is produced from all air alike, that what immediately surrounds the earth is not mere air, but a sort of vapor, and that its vaporous nature is the reason why it condenses back to water again. But if the whole of that vast region is vapor, the amount of air and of water will be disproportionately great. For the spaces left by the heavenly bodies must be filled by some element. This cannot be fire, for then all the rest would have been dried up. Consequently, what fills it must be air and the water that surrounds the whole earth-vapor being water dissolved. After this exposition of the difficulties involved,

let us go on to lay down the truth, with a view at once to what follows and to what has already been said. The upper region as far as the moon we affirm to consist of a body distinct both from fire and from air, but varying degree of purity and in kind, especially towards its limit on the side of the air, and of the world surrounding the earth.

Now the circular motion of the first element and of the bodies it contains dissolves, and inflames by its motion, whatever part of the lower world is nearest to it, and so generates heat. From another point of view we may look at the motion as follows. The body that lies below the circular motion of the heavens is, in a sort, matter, and is potentially hot, cold, dry, moist, and possessed of whatever other qualities are derived from these. But it actually acquires or retains one of these in virtue of motion or rest, the cause and principle of which has already been explained.

So at the centre and round it we get earth and water, the heaviest and coldest elements, by themselves; round them and contiguous with them, air and what we commonly call fire. It is not really fire, for fire is an excess of heat and a sort of ebullition; but in reality, of what we call air, the part surrounding the earth is moist and warm, because it contains both vapor and a dry exhalation from the earth. But the next part, above that, is warm and dry. For vapor is naturally moist and cold, but the exhalation warm and dry; and vapor is potentially like water, the exhalation potentially like fire.

24. After showing that neither fire nor air is the heavenly body called "first element" or "first body," he now intends to show how fire and air are related to that first body. Concerning this he does two things:

first, he raises this question, and two others necessary for his proposition;

second, he answers them, at: let us go on (340b4).

The first is divided into three parts, according to the three questions raised:

the second begins at and the reason why (340a21);

the third at let us first treat of the air (340a22).

He says therefore first that after the above considerations there remains to be examined the relation of air and of fire to the first body, namely, the heavenly body, since it has been shown that it is something other than they.

25. Then when he says, and the reason why (340a21), he places the second question, namely, as to what cause is due the fact that from the upper stars heat is produced in the regions surrounding earth. This question, too, takes its rise from the foregoing considerations. For it seems to be according to nature that like should generate like: if, therefore, the heavenly body is not hot—for it is neither fire nor air, as was shown above—then there remains the problem of how heat can be caused by the heavenly body in these lower bodies.

26. Then at let us first treat of the air (340a22), he raises the third problem which also arises from the foregoing. For he had said previously that we must inquire how the nature of air in the universe is to be taken, and this for the reason that many of the things concerning which he is about to determine are generated in the air. He says therefore that, as we laid down above, we must first speak of air; then we shall have to discuss the other two questions proposed. Hence he immediately begins to raise the difficulty on the nature of air.

For it has been shown in On Generation II that water is produced from air and conversely. Thus, since the rains are generated from the condensings of the clouds, this is air being converted into water. He asks, therefore, if water comes to be from air, and air from water, why is it that, in the upper region of air, clouds are not condensed [thickened] to generate water? And he gives a reason to show that such a thing should happen. For it is plain that condensation of clouds is due to coldness, for just as heat causes things to rarify, so cold causes things to condense. Now it seems that a place should be colder the farther it is from earth, because in such a place the two causes of heating are lacking.

One of these causes is proximity to the stars that cause heat; and this is referred to when he says that the upper region of air is not close enough to the stars which are hot, namely, as to effect, to allow the heat of the stars to prevent the thickening of clouds.

The other cause of heat is solar rays bouncing back from the earth; this cause he refers to when he says that neither is that region of upper air near the rays refracted, i.e., reverberated, from the earth, which rays prevent clouds from gathering close to the earth, inasmuch as their heat breaks down the consistency of the vapors. That this second cause does not prevent congregation he shows through a sign. For it is plain that the comings together of clouds occur where the rays reflected from the earth have already lost their power to heat through being immeasurably dispersed and are thus far distant from the rays that strike the earth; hence the cause of heat is not multiplied.

This last statement becomes plain if we keep in mind the fact that the rays coming from the sun to the earth are what cause heat. When a ray which comes from the sun to earth is reflected, there is now produced another ray as though moving upwards from the earth. The closer these two rays are to each other, the greater the heat produced, because the powers of both rays, i.e., that of the incident ray and that of the reflected ray, act on the same part of the air.

Consequently, where a ray of the sun strikes the earth at a right angle, there a maximum of heat is produced, because the ray is reflected into the same direction; when a ray falling on some place strikes at an angle less than a right angle, the heat is by that much less, since, as repercussion takes place according to similar angles, a reflected ray, because of the wideness of the angle, is greatly distant from the ray which first falls. It is plain that the farther two lines containing an angle proceed, the greater becomes the distance between them.

Hence, the farther one moves from the earth, where reflexion takes place, the farther the aforesaid two rays grow distant from each other, and there is less heat. Therefore, due to the immense separation of the aforesaid rays from one another in the upper region, the heat ceases, and clouds are formed there on account of the cold. And this what he says: gatherings of clouds are produced where the rays now cease because of immeasurable scatterings.

Consequently, both causes that could prevent forming of clouds are absent in the upper region of air, as has been said. But since, notwithstanding this, clouds do not form there, it is necessary to spy either that water cannot be formed from just any air, or, if all air is alike so far as being convertible into water is concerned, this air near the earth must not only be air, but as a vapor, and for this reason be gathered to generate water, while the upper air, which is pure air, cannot be condensed into water.

But this cannot be: for if the totality of air near the earth, since it is such a great amount, is vapor, then it is seen to follow that the nature of air and of water would far exceed the other elements. Because the higher distances, between the stars, are filled with some body (for there is no such thing as a void, as was proved in Physics IV, and they cannot be filled with fire, for this would result in everything's drying up, as was proved above, consequently they must be filled with air, while the region near the earth is filled with water. But this air is vaporous, for vapor is a certain disjoining of water, i.e., rarefied water. Then, having presented the three questions, he adds, as though summarizing, that the problems concerning the aforesaid have been raised in this manner.

27. Then when he says, let us go on (340b4), he solves the questions proposed:

first, the question about the order of the elements;

second, the one about generation of clouds, at so we must take the reason (340b29);

third, the one about heat produced in lower bodies by the stars, at as for the heat (341a12).

Regarding the first he does three things. First, he repeats what has been said about the nature of the first body and says that for an understanding both of the matters now in question and of matters to be stated later, we must state decisively that the highest body as far as the moon is other than fire and air, as has already been proved, and that there is in that highest body something more pure, and something less pure or sincere, without implying that there is present any composition or mixture of an extraneous nature. Rather "more pure" implies more noble, more virtuous, more formal. Hence it admits of differences both in power and in nobility. This difference is especially evident where it ceases at the air and the lower world surrounding the earth, for in the moon there appear defects of light, and, when it is full, certain dark areas appear.

28. second, at now the circular motion of the first element (340b10), he shows what effect the higher body has upon the lower ones. And he says that by means of the first element, i.e., the heaven, circularly moved, and the bodies moved in it, namely, sun and stars, that part of the lower world nearest to it, dispersed or rarefied, as it were, by the motion of the superior body, becomes inflamed and heat is produced.

And he gives the reason, saying that to understand this we must go back to the beginning. For the entire bodily nature that exists under the circularly moved body is as a certain matter which is in potency to heat, cold, dryness, and wetness, and to the other passions and forms that result therefrom; and because matter is reduced to act by the first agent, bodily nature also becomes actually such and such by the fact that it participates in the motion, or does not participate but remains immobile, from the heavenly body, which we have previously declared to be the cause and originative principle of the motion in these lower bodies.

This does not mean that the lower bodies receive such passions from the superior bodies, as it were, incidentally, and not according to nature, as when water becomes heated by fire; rather, the very nature or form according to which they are naturally hot or cold is received more principally from the superior body than from their generator, for the first principle of generation is the heavenly body.

third at so at the centre (340b19), he shows the order of the elements. For if heat originates in these lower bodies through participation in motion, and if, on the other hand, cold is due to distance from the heavenly motion, then, of necessity, that which is coldest and heaviest, namely, water and earth, is more removed from the heavenly motion and exists in the middle, as the earth does, or about the middle, as water does. Or else he says, around the middle, because the middle, being indivisible, cannot be the place of a body, but rather, earth and water are around the middle, i.e., the center of the world—for the center of the earth is in the center of the whole. Around these, namely, earth and water, and had to these, i.e., ordered in sequence after them, are air, and what is commonly called fire, in which [two] heat abounds.

He explains his statement that the fourth element, i.e., the one located above air, is not strictly called "fire." For "fire" signifies an excess of heat and is, as it were, a certain intensity and igniting. In the same way, ice is not an element but a certain superabundance of coldness producing congealed water. Now that to which fire is related in the way that ice is related to water has no name; so we call it by the name of fire. It is as though water should have no name and we should call the element of water, "ice." But we must understand that in regard to that whole body we call "air," one part, the part nearest the earth, is as though hot and moist on account of vapor, and exhalation from the earth.

For the elements are arranged in a manner that befits their nature; therefore, because air is naturally hot and moist, it is disposed to receive vapor from the earth to preserve its heat and moisture. But that part of the body commonly called "air" which is higher, is hot and dry; and this upper part we call the element "fire." In this way the name "air" is common to two elements. And because he had spoken about vapor and exhalation from the earth, he shows the difference between them and says that the nature of vapor is to be moist and hot, whereas the nature of an exhalation is to be hot and dry. As a result, vapor is, on account of its moistness, in potency to water; but an exhalation, on account of its dryness is, as it were, in potency to be ignited.