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.(a) 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.(b) 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'.(c) In yet another sense we give the name to all body included within extreme circumference, since we habitually call the whole or totality 'the heaven'.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.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.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 centre or that which lies lowest.Naturally they could not be there, since their proper places are elsewhere; 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.
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.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?' 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.
It is therefore evident that there is also no place or void or time outside the heaven.For in every place body can be present; and void is said to be that in which the presence of body, though not actual, is possible; and time is the number of movement.But in the absence of natural body there is no movement, and outside the heaven, as we have shown, body neither exists nor can come to exist.
It is clear then that there is neither place, nor void, nor time, outside the heaven.Hence whatever is there, is of such a nature as not to occupy any place, nor does time age it; nor is there any change in any of the things which lie beyond the outermost motion; they continue through their entire duration unalterable and unmodified, living the best and most selfsufficient of lives.As a matter of fact, this word 'duration' possessed a divine significance for the ancients, for the fulfilment which includes the period of life of any creature, outside of which no natural development can fall, has been called its duration.On the same principle the fulfilment of the whole heaven, the fulfilment which includes all time and infinity, is 'duration'-a name based upon the fact that it is always-duration immortal and divine.From it derive the being and life which other things, some more or less articulately but others feebly, enjoy.So, too, in its discussions concerning the divine, popular philosophy often propounds the view that whatever is divine, whatever is primary and supreme, is necessarily unchangeable.This fact confirms what we have said.For there is nothing else stronger than it to move it-since that would mean more divine-and it has no defect and lacks none of its proper excellences.Its unceasing movement, then, is also reasonable, since everything ceases to move when it comes to its proper place, but the body whose path is the circle has one and the same place for starting-point and goal.
