The Six Books of Proclus on the Theology of Plato, Volume 2

Every being is either immoveable or moved. And if moved, it is either moved by itself, or by another. And if indeed it is moved by itself, it is self-motive; but if by another, it is alter-motive. Every thing therefore, is either immoveable, or self-motive, or alter-motive.

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.