(CG 3/14). That Evil is an Accidental Cause (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

A sovereign evil should be without participation in any good, as that is the sovereign good which is wholly removed from evil. But there cannot be any evil wholly removed from good, since evil is founded on good (Chap. XI).

2. If anything is sovereignly evil, it must be evil by its very essence, as that is sovereignly good which is good by its essence. But evil has no essence (Chap. VII).

3. That which is a first principle is not caused by anything. But all evil is caused by good (Chap. X). There is therefore no evil first principle.

5. The incidental must be posterior to the ordinary. But evil happens only incidentally and beside the intention (Chap. IV). Therefore it is impossible for evil to be a first principle.

Hereby is excluded the error of the Manicheans.