From these arguments some one may believe, that there is a certain first immoveable nature; and again, if he looks to the principles of motive natures. That there are some things, therefore, which at one time are moved, and at another are at rest, is evident. And from this it becomes manifest, that neither are all things moved, nor all things at rest, nor that some things are always at rest, and others are always moved; for things which verge to both these, and possess a power, at one time of being moved, and at another of being at rest, afford a demonstration concerning these. Since, however, things of this kind are manifest to all men, and we also wish to show each nature of the two, viz. that some things indeed are always immoveable, but others always moved, and since we have proceeded as far as to this, and have established as principles, that every thing which is moved, is moved by something, and that this is either immoveable or moved, and is always moved either by itself or by another, we have at length assumed, that the principle of tbings which are moved, is that which itself moves itself, itself, and that the principle of all things is that which is immoveable. But we see indeed, and evidently, that there are things of such a nature as to move themselves; as the genus of things animated, and of animals. These also afford an opinion, that motion might happen to be generated, when, in short, it was not, because we see this takes place in animated natures; for these, as it seems, when thay have been without motion, are again moved. This then it is necessary to assume, that these move themselves according to one motion, and according to this not properly; for the cause is not from the animal, but other natural motions are inherent in animals, according to which they are not moved through themselves; such as augmentation, diminution, and respiration, with which every animal is moved when at rest, and is not moved according to the motion which is produced from itself. But the cause of this is that which contains, and many of the things which enter into the animal. Thus, for instance, food is the cause of some motions: for when it is concocted animals sleep, and when it is distributed they awake, and move themselves, the first principle of motion being external. Hence they are not always continually moved by themselves; for to each of the natures which move themselves, something else is present, which moves, being itself moved and changed. In all these, however, that which first moves, and is the cause of a thing moving itself, is also moved by itself, but according to accident: for the body changes its place; and therefore that which is in the body, and which moves itself by a lever. From hence it may be believed, that if there is any thing which ranks among things immoveable indeed, but which move and are themselves moved according to accident, it is impossible that it can move with a continued motion. So that if it is necessary, that there should be continued motion, it is also necessary that there should be some first nature, which moves being immoveable, and this not according to accident; if there ought to be, as we have said, in things, a certain unceasing and immortal motion, and if being ought to abide in itself, and in the same: for the principle remaining, it is also necessary that the universe should remain, since it is in continuity with the principle. It is not, however, the same thing, for any being to be moved accidentally by itsdf and by another; for to be moved by another, belongs also to some of the principles in the heavens, viz. to some of those which are moved with many lations; but the other belongs to corruptible natures alone.
