De principiis naturae

Having seen, then, that there are four causes, namely the efficient, material, formal, and final, it should be known that each of these causes is divided in many ways. For one thing is said to be a prior cause and another a posterior cause: as we say that art and the doctor are the cause of health, but art is the prior cause and the doctor the posterior cause; and likewise in the formal cause and in the other causes.

And note that we must always trace the question back to the first cause; as if it is asked "Why is this man healthy?", it is to be answered "Because the doctor healed him"; and again "Why did the doctor heal him?", "On account of the art of healing which he possesses."

It should be known that to say "proximate cause" is the same as to say "posterior cause," and "remote cause" the same as "prior cause"; hence these two divisions of causes, the one into prior and posterior causes and the one into remote and proximate causes, signify the same thing. This, however, is to be observed: that what is more universal is always called the remote cause, and what is more particular the proximate cause: as we say that the proximate form of man is his definition, namely rational mortal animal, but animal is more remote, and again substance still more remote. For all higher things are forms of the lower. And likewise the proximate matter of a statue is copper, but the remote matter is metal, and still more remote, body.

Again, of causes one is per se and another per accidens. A per se cause is said to be the cause of a thing insofar as it is of that kind, as the builder is the cause of the house and wood the matter of the bench. A per accidens cause is that which is incidental to the per se cause, as when we say "The grammarian builds"; for the grammarian is said to be the cause of the building per accidens, for it is not insofar as he is a grammarian but insofar as it is incidental to the builder. And so it is in the other causes.

Again, of causes one is simple and another composite. A cause is called simple when only that which is per se the cause is said to be the cause, or even only that which is per accidens: as if we say that the builder is the cause of the house, and likewise if we say that the doctor is the cause of the house. But it is called composite when both are said to be the cause, as if we say "The doctor-builder is the cause of the house." A cause can also be called simple, as Avicenna explains, in the sense of that which is a cause without the addition of another, as copper is of the statue, for without the addition of any other matter the statue is made from copper; and thus it is said that the doctor produces health, or that fire heats. But a cause is composite when several things must concur for it to be a cause, as one man alone is not the cause of the movement of a ship, but many; and as one stone is not the matter of a house, but many.

Again, of causes one is in act and another in potency. A cause in act is one that actually causes the thing, as the builder when he is building, or copper when the statue is from it; but a cause in potency is one which, although it does not actually cause the thing, can nevertheless cause it, as the builder when he is not building. And it should be known that, speaking of causes in act, the cause and the caused must exist together, so that if one exists, so does the other: for if there is a builder in act, he must be building, and if there is building in act, there must be a builder in act. But this is not necessary in causes that are only in potency.

It should be known, moreover, that a universal cause is compared to a universal effect, and a singular cause to a singular effect: as we say that a builder is the cause of a house, and this builder of this house.