That there are causes, therefore, and that they are as many in number as we have said, is evident: for the inquiry why a thing is comprehends so many in number: for the question why a thing is, is either referred to essence, which is an extreme in things immovable, as in the mathematics; since the question here is ultimately referred to the definition of a right line, or of the commensurate, or something else. Or it is referred to the first mover; as, for instance, why did they fight? Because they have been robbed. Or, for the sake of what? That they might obtain dominion. Or, cause is, in things which are generated, matter. That these are causes, therefore, and that they are so many, is evident. But since causes are four in number, to know them all is the business of the natural philosopher, who also referring the cause why a thing is to all of them, namely, to matter, form, that which moves, and that 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 whence motion first originates, is in species the same with these: for man generates man; and, in short, such things as being moved, move. But such things as do not move, being moved, no longer pertain to physical inquiries: for they move, not in consequence of containing in themselves motion, or the principle of motion, but as being immovable. Hence there are three treatises; one concerning that which is immovable; another concerning that which is moved, indeed, but is incorruptible; and a a third concerning corruptible natures. So that the cause why a thing is, is assigned by him who refers to matter, to essence, and to the first mover: for concerning generation, they especially consider causes after this manner, what is generated after what, and what a thing first did or suffered, and thus always in a consequent order. 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 anything which moves without being moved, it is of this kind; as is that which is perfectly immovable, 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. The cause, two, why a thing subsists must be entirely assigned: for instance, it is necessary this thing should be produced from that; and this either simply or for the most part. It is also necessary to know if this particular thing will follow, as, for instance, the conclusion from the propositions: also that this is the essence of a thing; And because it is thus better, not simply, but with reference to the essence of each particular.
