"[S]ince causes are four in number, to know them all is the business of the natural philosopher, who also referring to the cause why a thing is to all of them, viz. to matter, form, that which moves, and for the sake of which a thing subsists, physically assigns a reason. Frequently, however, three of these causes pass into one: for the cause why a thing is, and that for the sake of which it is, are one. But that which motion first originates, is in species the same with these... [T]here are three treatises; once concerning that which is immoveable; another concerning that which is moved, indeed, but is incorruptible; and a third concerning corruptible natures. So the cause of why a thing is, is assigned by him who refers to matter, to essence, and to the first mover... But there are two principles which are naturally motive; of which, one is not physical, because it does not contain in itself the principle of motion. And if there is any thing which moves without being moved, it is of this kind; as is that which is perfectly immoveable, that which is the first of all things, together with essence and form: for it is the end, and that for the sake of which a thing subsists. So that since nature is for the sake of something, it is also necessary to know this cause."