Commentaria in Summam contra Gentiles, Liber III

From all the aforesaid, St. Thomas deduces that there cannot be any highest evil that is the principle of all evils ¹.

And he argues first as follows. There cannot be any evil entirely separated from good. Therefore there is no highest evil. — The consequence is clear. Because the highest evil must be without the companionship of any good, just as the highest good is entirely separated from all evil.

Secondly. Evil has no essence. Therefore, etc. — The consequence is clear. Because what is supremely evil must be evil through its essence, just as what is supremely good is good through its essence.

Note that just as good is said both through essence and through participation, so, if a highest evil were granted, evil would have to be divided into evil through essence and evil through participation. But the highest evil would not be evil through participation; otherwise it would not be the highest evil. Therefore it would be necessary that it be evil through its essence, as St. Thomas assumes.

Thirdly. Every evil is caused by a good. Therefore evil is not a first principle, since a first principle is not caused by anything.

To this argument it could be said that, although evil is not a first principle simpliciter, there is nonetheless some evil that is the first principle of all evils. But this response is invalid, because, as is clear from the above ², even in the causation of evil a resolution would have to be made into a good, as is also touched upon in the following argument.

Fourthly. Evil does not act except in the power of a good. Therefore it cannot be a first principle, since such a principle acts by its own power.

Fifthly. Evil does not occur except per accidens and contrary to intention. Therefore it is not first. Therefore it is not a first principle. — The first consequence is proved. Because what is per accidens is posterior to what is per se.

Note that those who posit that a highest evil is the principle of all evils thought that evil is its first and per se effect. And for this reason this argument is excellent against them. For if evil is not produced except per accidens and contrary to intention, as is evident from the above ³, it is clear enough that it cannot be intended primarily and per se. From which it follows that there is no first principle of evil—namely, as a first and per se cause of evil, as the Manichees maintained.

Sixthly. Every evil has a cause per accidens. Therefore it is not a first principle, since that does not have a cause either per se or per accidens.

Seventhly. Evil is not a cause except per accidens. Therefore it is not a first principle, because a cause per se is prior to a cause per accidens.

From these points is excluded the error of the Manichees, who posit a highest evil that is the principle of all evils.