Since circular motion is not the contrary of the reverse circular motion, we must consider why there is more than one motion, 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.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.
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 centre of the revolving body; and of that body no part can be at rest, either elsewhere or at the centre.It could do so only if the body's natural movement were towards the centre.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 centre.(At present we may take this for granted: it shall be explained later.) But if earth must exist, so must fire.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.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.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.) these four elements generation clearly is involved, since none of them can be eternal: for contraries interact with one another and destroy one another.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.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: 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.
