Chapter 14 — That evil is an accidental cause

From the same premises it is clear that although evil is not a cause in itself, it is nevertheless an accidental cause.

For if A is the cause of B in itself, whatever is accidental to A is the accidental cause of B. Thus white, which is accidental to the builder, is the accidental cause of the house. Now every evil is in some good. (Ch. 11.) And every good is in some way the cause of something, for matter is in some way the cause of form, and in some way the reverse is true; the same applies to the agent and the end. Therefore, there does not follow an indefinite sequence in causes if each thing be the cause of something else, because the circle to be observed in causes and effects is composed of various kinds of cause. Therefore, evil is an accidental cause.

Again. Evil is a privation, as shown above. (Ch. 7.) Now privation is an accidental principle in movable things, even as matter and form are essentially principles. Therefore, evil is an accidental cause of something.

Moreover. From a defect in the cause there follows defect in the effect. Now defect in a cause is an evil. And yet it cannot be a cause in itself, since a thing is not a cause in that it is defective, but in that it is a being, since if it were wholly defective, it would be the cause of nothing. Therefore, evil is a cause of something not in itself, but accidentally.

Further. If we run through all the kinds of cause, we find that evil is an accidental cause. It is so in the species of efficient cause, because defect in effect and action results from a defect in the efficient cause. It is so in the species of material cause, because a fault in the effect arises from indisposition in the matter. It is so in the species of formal cause, because every form is accompanied by the privation of the opposite form. And it is so in the species of final cause, because evil is united to the undue end, inasmuch as the due end is hindered thereby.

It is therefore evident that evil is an accidental cause, and cannot be a cause essentially.