That the Variety of Creatures does not arise from any Contrariety of Prime Agents*

IF the diversity of things proceeds from diversity or contrariety of diverse agents, this would seem to hold especially of the contrariety of good and evil, so that all good things should proceed from a good principle, and evils from an evil principle. Now there is good and evil in all genera. But there cannot be one first principle of all evils: for the very essence of such a principle would be evil, and that is impossible. Everything that is, inasmuch as it is a being, must necessarily be good: for it loves and strives to preserve its own being, a sign whereof is this fact, that everything fights against its own destruction: now what all things seek is good. It is impossible therefore for the diversity of things to arise from two principles, one good and one evil.*

9. What in no manner of way is, is neither good nor evil: while every thing that is, in so far as it is, is good. A thing can be evil therefore only inasmuch as it is not-being, that is, privative being;* and the evil is precisely the privation. Now privation never comes of the ordinary action of any cause: because every cause acts inasmuch as it is endowed with 'form'; and thus the ordinary effect of its action must also be endowed with 'form,' since every agent acts to the production of its own likeness, unless it be accidentally hindered. It follows that evil does not come of the ordinary action of any cause, but is accidentally incident among the effects of ordinary causation.* There is therefore no one primary and essential principle of all evil: but the first principle of all is one primary good, among the effects of which there ensues evil incidentally.

Hence it is said: I am the Lord, and there is none other, forming light and creating darkness, making peace and creating evil: I am the Lord doing all these things (Isa. xlv, 6, 7). And, Good things and evil things, life and death, poverty and rank are from God (Ecclus xi, 14). And, Against evil is good, and against life death; so against the just man is the sinner. And so behold all the works of the Most High, two and two, and one against one (Ecclus xxxiii, 15).

God is said to make and create evil things, inasmuch as He creates things that are good in themselves and yet hurtful to others: thus the wolf, though a good thing naturally in his kind, is evil to the sheep. Hence it is said: Shall there be evil in the city that the Lord hath not done? (Amos iii, 6.)

Hereby is excluded the error of those who suppose two primitive contrary principles, good and evil. This error of the early philosophers some evil-minded men have presumed to introduce into Christian teaching, the first of whom was Marcion, and afterwards the Manicheans, who have done most to spread this error.

2.38 : Arguments wherewith some try to show that the World is not Eternal, and Solutions of the same
2.44 : That the Variety of Creatures has not arisen from Variety of Merits and Demerits