But it happens reasonably, that the motion in a circle is one and continued, and not that which is in a right line: for of the motion which is in a right line, the beginning, middle, and end are bounded, and it contains all these in itself; so that there is whence that which is moved began, and where it will end: for every thing rests in boundaries; either from whence, or whither it is moved; but these in a circular motion are indefinite: for why is any one of those things which are in a circular motion the boundary rather than the rest? Since each is similarly beginning, middle, and end; so that some things are always and never in the beginning and end. Hence a sphere, in a certain respect, is moved and is at rest: for it occupies the same place. But the reason of this is that all these things happen to the center: for it is the beginning, middle, and end of magnitude. Hence, since this is external to the circumference, there is not any place where that which is moved can rest, as if it had finished its motion; for it is always carried about the middle, and not to the extremity. On this account the whole in a certain respect, always remains and rests, and yet is continually moved. But this happens reciprocally: for because circulation is the measure of motions, it is necessary that it should be the first motion: for all things are measured by that which is first; and because it is first, it is the measure of others. Farther still, the motion alone in a circle can be equable: for things which are moved in a right line, are moved anomalously from the beginning, and to the end; for all things, by how much the farther they are distant from that which is at rest, are moved by so much the swifter; but in a circular motion alone, neither the beginning nor the end are naturally adapted to be in it, but are external. Indeed that lation according to place is the first of motions, is testified by all those who have made mention of motion: for they attribute the principles of it to those things which produce this motion: for separation and concretion are motions according to place. Thus too, friendship and strife move: for the latter of these separates, and the former congregates. And Anaxagoras says that intellect which is the first mover separates; and they alto think similarly, who say that there is no such cause as this, but that things are moved through a vacuum: for these also assert that nature is locally moved; since motion through a vacuum is lation, and is as it were in place. But they fancy that no other motion is in first natures, but in those things which are composed from these: for they say that composite natures are increased, diminished, and changed in quality, in consequence of atomic bodies being congregated and separated. Those also think after the same manner, who produce generation and corruption through density or rarity; for they orderly dispose these by concretion and separation. To these also those may be added, who make soul the cause of motion: for they say that which itself moves itself, is the principle of things which are moved; but animal, and everything animated, moves itself with local motion. We also say, that a thing which is locally moved is alone properly moved. But if it rests indeed in the same place, but is increased or diminished, or changed in quality, we say that it is in a certain respect moved, but is not simply moved. That motion, therefore, always was, and will be through the whole of time; what the principle also is of perpetual motion; and farther still, what the first motion is, and what motion can alone can be perpetual, and that the first mover is immovable, has been said.
