For it is necessary since there are alter-motive natures, that there should also be that which is immoveable, and that the self-motive nature should subsist between these. For if every thing alter-motive is moved in consequence of being moved by another thing, motions will either be in a circle, or they will proceed to infinity.[212] But they will neither be in a circle, nor have an infinite progression, since all beings are bounded by the principle of things and that which moves is better than that which is moved. Hence there will be something immoveable which first moves.[213] But if this be the case, it is also necessary that there should be something which is self-motive. For if[214] all things should stop, what will that be which is first moved? It cannot be that which is immoveable;[215] for it is not naturally adapted to be moved; nor that which is alter-motive; for that is moved by something else. It remains therefore, that the self-motive nature is that which is primarily moved. For it is this also which conjoins alter-motive natures to that which is immoveable, being in a certain respect a middle, moving and at the same time being moved. For of these, the immoveable moves only, but the alter-motive is moved only. Every thing therefore, is either immoveable, or self-motive, or alter-motive. COROLLARY. From these things likewise, it is evident, that of things which are moved, the self-motive nature is the first; but that of things which move the immoveable is the first. PROPOSITION XV. _Concerning an incorporeal essence, and what the peculiarity of it is._ Every thing which is converted to itself is incorporeal. * * * * * For no body is naturally adapted to revert to itself. For if that which is converted to any thing is conjoined with that to which it is converted, it is evident that all the parts of the body which is converted to itself, will be conjoined with all the parts. For this it is for a thing to be converted to itself, when both that which is converted, and that to which it is converted, become one. This however is impossible in body, and in short, in all partible things. For the whole of that which is partible is not conjoined with the whole, on account of the separation of the parts, some of which are situated differently from others. No body therefore, is naturally adapted to revert to itself, so as that the whole may be converted to the whole. Hence if there is any thing which has the power of reverting to itself, it is incorporeal and impartible. PROPOSITION XVI. Every thing which is converted to itself, has an essence separate from all body. * * * * *