Commentaria in Summam contra Gentiles, Liber III

After St. Thomas determined concerning the causality of evil with respect to its being caused, he now determines concerning its causality with respect to its being the cause of other things ¹.

And he posits this conclusion: Evil, although it is not a cause per se, is nevertheless a cause per accidens.

I. It is proved first. Evil is in good. Therefore it is a cause per accidens. — The consequence is proved. Because what happens to the cause per se of something is the cause per accidens of that thing. But every good is in some way the cause of something: as matter is in a certain way the cause of form, and conversely; and similarly concerning the agent and the end. And from this an infinite regress in causes and things caused is eliminated, if everything is the cause of something.

Concerning this argument, it should be noted that an accidental cause, according as the accidentality holds on the part of the cause, effects nothing toward the effect: as, when a white builder builds, whiteness, which is the cause per accidens of the building, effects nothing toward the building, but is said to cause per accidens because the subject to which it happens, and which is denominated by it, causes; for it is true to say that a white thing builds, because that which underlies the whiteness builds. Similarly, therefore, evil is said to be a cause per accidens, not because it produces anything regarding the effect of good, but because the acting supposit underlies malice. Hence when it is said, evil acts, it is true: because that which underlies malice, and is denominatively called evil, acts.

It should also be noted that St. Thomas added that an infinite regress in causes and things caused does not follow if everything is the cause of something, in order to remove the objection that could be made against this assumed proposition, Every good is in some way the cause of something. For an infinite regress in causes and things caused seems to follow in descending: because if one does not proceed to infinity, it is necessary to arrive at some good that is caused and in no way a cause. He answers by denying the consequence: because one never arrives at something that is in no way a cause, and yet one does not proceed to infinity, because what is the effect of one thing in one genus of cause can be the cause of that thing in another.

II. Secondly. Privation is a cause per accidens in movable things. Therefore etc. — The consequence is clear. Because evil is a certain privation: — (supply) and every privation is a certain evil.

Note that privation, which in I Physics ² is counted along with matter and form in the number of natural principles, inasmuch as it is conjoined to matter, is not a principle of things except inasmuch as they are changeable. For according as some things are changeable, so privation is their principle. For if some things are substantially changeable, as are lower bodies, so the privation of substantial form is its principle per accidens, inasmuch as to its matter is conjoined the privation of another substantial form which it is naturally apt to have: as to the matter of water is conjoined the privation of the form of fire. But if something is accidentally changeable, the privation of accidental form is its principle per accidens as it is thus changeable, inasmuch as to its substance is conjoined the privation of another accidental form: as to a white subject, changeable to blackness, is conjoined the privation of blackness. On account of this, St. Thomas did not say absolutely that privation is a principle per accidens in things, but in movable things.

III. Thirdly. A defect of the cause is the cause per accidens of a defect in the effect. And it is a certain evil. Therefore etc. — It is proved that it is a cause per accidens. Because a thing is not a cause through this, that it is deficient, but through this, that it is a being. The sign of which is that, if it were entirely deficient, it would be the cause of nothing.

Note that a formal cause per se is that by which something acts, and by which it elicits an operation. But this can only be a being: because nothing acts except inasmuch as it is in act. And therefore it is exceedingly well proved that evil taken formally is not a cause per se, because a thing is not a cause through this, that it is deficient: that is, defect and privation is not that by which it acts and by which it elicits an operation.

IV. Fourthly. On account of the deficient power of the efficient cause a defect in action and effect follows. On account of the indisposition of the matter a defect in the effect is caused. To one form is always attached the privation of another form: taking privation broadly, that is. Evil is always conjoined to an undue end, inasmuch as through it the due end is impeded. Therefore according to all species of causes evil is a cause per accidens.