Summa contra Gentiles3.14 Chapter XIV. That evil is an accidental cause

From the same premises it is clear that although evil is not a per se cause, it is nevertheless an accidental cause. For if A is the cause of per se, whatever is accidental to A is the accidental cause of B: thus whtte which is accidental to the builder, is the accidental cause of the house. Now every evil is in some good. And every good is in some way the cause of something: for matter is in some way the cause of form; while the converse is true in a sense: and the 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. Now an accidental cause of something.

Moreover. From a defect in the cause there follows defect in the effect. Nowdefect inacauseisanevil. And yet it cannot be a per se cause; since a thing is not a cause in that it is defective, but in that it is a being: since were it wholly defective, it would be the cause of nothing. Therefore evil is a cause of something, not per se, but accidentally.

Further. If we run through all the kinds of cause, we find that evil is an accidental cause. In the species of efficient cause, because defect in effect and action results from a defect in the efficient cause. In the species of material cause, because a fault in the effect arises from indisposition in the matter. In the species of formal cause, because every form is accompanied by the privation of the opposite form. And 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 per se.