Lecture 20 — The universe shown to consist of every natural and sensible body as its matter

137 It remains, then, only to prove that it is composed of all natural perceptible body. First, however, we must explain what we mean by 'heaven' and in how many senses we use the word, in order to make clearer the object of our inquiry.

138 In one sense, then, we call 'heaven' the substance of the extreme circumference of the whole, or that natural body whose place is at the extreme circumference. We recognize habitually a special right to the name 'heaven' in the extremity or upper region, which we take to be the seat of all that is divine. In another sense, we use this name for the body continuous with the extreme circumference which contains the moon, the sun, and some of the stars; these we say are 'in the heaven'. In yet another sense we give the name to all bodies included within extreme circumference, since we habitually call the whole or totality 'the heaven'.

139 The word, then, is used in three senses. Now the whole included within the extreme circumference must be composed of all physical and sensible body, because there neither is, nor can come into being, any body outside the heaven.

140 For if there is a natural body outside the extreme circumference it must be either a simple or a composite body, and its position must be either natural or unnatural.

141 But it cannot be any of the simple bodies. For, first, it has been shown that that which moves in a circle cannot change its place. And, secondly, it cannot be that which moves from the center or that which lies lowest. Naturally they could not be there, since their proper places are elsewhere;

142 and if these are there unnaturally, the exterior place will be natural to some other body, since a place which is unnatural to one body must be natural to another: but we saw that there is no other body besides these. Then it is not possible that any simple body should be outside the heaven.

143 But, if no simple body, neither can any mixed body be there: for the presence of the simple body is involved in the presence of the mixture.

144 Further neither can any body come into that place: for it will do so either naturally or unnaturally, and will be either simple or composite; so that the same argument will apply, since it makes no difference whether the question is 'does A exist?' or 'could A come to exist?'

145 From our arguments then it is evident not only that there is not, but also that there could never come to be, any bodily mass whatever outside the circumference. The world as a whole, therefore, includes all its appropriate matter, which is, as we saw, natural perceptible body. So that neither are there now, nor have there ever been, nor can there ever be formed more heavens than one, but this heaven of ours is one and unique and complete.

198. Having presented the solution brought forward, the Philosopher here proves what he had presupposed, namely, that the world consists of all its matter.

First he tells his intention and order of procedure (278b8) and says that in order to complete the preceding solution, we must show that the world consists of every natural and sensible body, which is its matter. But before showing this, it is necessary to explain what is meant by this word heaven, and in how many senses it is used, so that our question can be answered more clearly.

199. Secondly, at in one sense, then, he proves his proposition:

first, he shows the various senses of the word heaven;

secondly, he proves the main proposition, at the word, then, is used in three (278b21; [200]).

With regard to the first he gives three senses of heaven. In one way the heaven is called the substance of the extreme circulation of the whole (278b11), i.e., that which is at the boundary of the whole universe and is moved circularly. And because he had explained the meaning of the word in terms of substance, whose notion transcends natural philosophy, since it pertains to Metaphysics, he adds another explanation having the same meaning, saying that the heaven is the natural body whose place is at the extreme circumference of the world, which explanation is more befitting to natural science. He proves this meaning from the way people speak—since words are to be used in the sense most people use them, as is said in Topics II. For men are more likely to call "heaven" that which is the extreme of the entire world and which is most up, not, indeed, as up is taken in natural science, i.e., as being the terminus of the motion of light things (for in this sense nothing is farther up than the place to which fire is borne) but as taken according to common parlance, where up designates that which is farther from the middle. Up also refers to the place of all divine beings (where "divine" signifies not celestial bodies—not all of which are in the outermost sphere—but non-material and incorporeal substances), for it has been said above that all men attribute to God a place that is up.

In a second way "heaven" means not only the outermost sphere but the whole body continuous with the extreme circumference of the whole universe, i.e., all the spheres of celestial bodies, in which exist the moon and sun and certain of the stars, namely, the other five planets (for the fixed stars are in the supreme sphere according to the opinion of Aristotle, who did not posit another sphere above that of the fixed stars). And he proves this meaning also on the basis of common parlance: for we say that the sun and moon and other planets exist in the heaven. Now these bodies are said to be continuous with the extreme sphere, because they are alike in nature, i.e., they are imperishable and movable circularly, and not because one continuous body is formed from all of them—for then they could not have several and different motions, a continuum being something whose motion is one, as is said in Metaphysics V.

In a third way heaven means the whole body contained within the extreme circumference, i.e., by the extreme sphere. This, too, he proves from the common use of the word—since we are wont to call the whole world and the totality, i.e., the universe, the heaven.

It should be noted that "heaven" is here used in these three ways not equivocally but analogically, i.e., in relation to one first. For it is the supreme sphere that is first and principally called "heaven"; secondly, the other celestial spheres from the continuity they have with the supreme sphere; thirdly, the universe of bodies insofar as they are contained by the extreme sphere.

200. Then, at the word, then, is used in three (278b21), he proves the proposition.

First he shows that there is no sensible body outside the heaven taken in the third sense, i.e., outside this world;

secondly, he shows that there is not outside it any of the things that are normally consequent upon natural bodies, at it is therefore evident (L. 21; 279a11).

As to the first he does three things:

first he proposes what he intends;

secondly, he proves his proposition, at for if there is (278b25; [201]);

thirdly, he concludes to his main proposition, at from our arguments then it is evident (279a6; [206]).

He says therefore first that whereas "heaven" is said in three ways, we shall be discussing it now in its third sense, where heaven is taken as the whole contained by the extreme circumference (278b21). Concerning this heaven it is necessary that it consist of every sensible and natural body—which is its matter, and thus it consists of all its matter—due to the fact that outside this heaven no body exists, nor can exist.

201. Then, at for if there is (278b25) he proves the proposition.

First he shows that there is no body outside the heaven;

secondly, that none can be there, at further neither can (279a2; [205]).

About the first he does two things:

first he presents a division through which he manifests the proposition;

secondly, he excludes each member of the division, at but it cannot be any of the simple (278b27; [202]).

He says therefore first (278b25) that if there is a physical, i.e., natural, body outside the extreme periphery, i.e., circumference, it has to be either of the number of simple bodies, or of the number of composite bodies. Moreover, it must exist there according to nature, or outside its nature.

202. Then, at but it cannot be any of the simple (278b27) he eliminates each member of this division.

First he shows that outside the extreme sphere no simple body exists according to nature. For simple bodies are such that one is moved circularly, one from the middle, and one is moved to the middle and in the middle supports all the others, as was had above. But none of these bodies can exist outside the extreme circumference. For it has been shown above in Physics VI that the circularly moved body does not as to its whole being change its place except in conception. Consequently, it is not possible for that body which is moved circularly to be transferred to a place outside of that in which it exists. But this would follow, if there were a circularly moved body existing outside the extreme circumference as in its natural place. Since the reason that it would be natural to that circularly moved body would also make it natural to the body circularly moved in this world, and every body is naturally borne to its natural place, it would follow, therefore, that that latter circularly moved body would be transferred outside its proper place to another place—which is impossible.

Similarly it is not possible for a light body which is moved from the center to be outside the extreme circumference or for a heavy body which supports the other bodies in the center. For if it is maintained that they exist naturally outside the extreme circumference, such a thing cannot be, since they have other natural places, namely, within the extreme circumference of the whole. For it was shown above that there is one numerical place for all heavy bodies and one for all light bodies. Hence it is not possible that those bodies be naturally outside the extreme circumference of the whole. And it should be noted that this argument, both as to the body circularly moved, and as to the body moved with straight motion, possesses necessity on account of what was proved above, namely, that there is but one extreme and one middle.

203. Secondly, at and if these are there unnaturally (278b32) he shows that no simple body is outside the heaven outside its nature. For if it were there in that manner that place would be natural to some other body; for a place outside nature for one body must be according to nature for some other—if a proper body were lacking to a place, that place would exist in vain. But it cannot be said that that place is natural to any body: for it is not natural to a circularly moved body, nor to a light or heavy body. But it has been shown above that there are no other bodies besides these. Consequently, it is plain that no simple body exists outside the heaven, either according to nature or outside nature.

204. Thirdly, at but, if no simple (278b35) he proves that there is no mixed body there. For if none of the simple bodies exists there, it follows that no mixed body is. Wherever there is a mixed body, simple bodies must be there, due to the fact that simple bodies are present in the mixed; and a mixed body gets its natural place according to the simple body predominant in it.

205. Then, at further neither can (279a2) he shows that outside the heaven there cannot be any body. Hence he says that it is not possible for a body to come to be outside the heaven. For it would be there either according to nature or outside nature; again, it would be either simple or mixed. But no matter which of these is given, the same situation as above would prevail. For according to the above-stated reasons, it makes no difference whether the question concerns the existence of a body outside the heaven, or the possibility of its coming to be there, since the foregoing arguments conclude both, and since in sempiternal things to be and to be able to be do not differ, as is said in Physics III.

206. Then, at from our arguments then it is evident (279a6) he draws the conclusion mainly intended. And he says it is manifest from what has been said that outside the heaven no mass of any sort of body exists, nor can exist, since the whole world consists of its entire proper matter and the matter of the world is the sensible natural body. However, it should be not understood that he wishes to prove that no sensible body exists outside the heaven on the ground that it consists of the totality of its matter; but rather the converse. Nevertheless, he uses that manner of speaking because the two are mutually convertible.

He concludes, therefore, that there are not many worlds at present, nor were there many in the past, nor will there ever be able to be in the future. Rather the heaven is one and unique and perfect in the sense of consisting of all its parts or of its total matter.