Done into syllogistic form, the argument might stand thus:
What all things seek, even a principle of evil would seek.
But all things seek their own self-preservation.
Therefore even a principle of evil would seek its own self-preservation.
What all things seek, is good.
But self-preservation is what all things seek.
Therefore self-preservation is good.
But a principle of evil would seek its own self-preservation.
Therefore a principle of evil would seek some good.
But a principle of evil ought to be averse to all good.
Therefore a principle of evil is absurd.
One wonders whether this is the argument that St Thomas thought of at the table of St Louis, when he suddenly started up and cried, Ergo conclusum est contra Manichaeos. But it is difficult to kill a heresy with a syllogism. One might perhaps distinguish between absolute and relative good; and upon that distinction urge that the self-preservation, which the evil principle sought, was good relatively to it only, but evil absolutely for the world.
The deepest flaw in the Manichean notion of an Evil Principle is that which is pointed out in the next argument (n. 9). Moreover every argument which establishes the unity and infinite perfection of God, is destructive of Manicheism. (Cf. Isaias xlv, 6, 7, quoted below.)
Matter is not evil, as Plato supposed, but its essential capacities for good are greatly limited; and, where good stops short, evil readily enters in. God does not override essentialities.
Of God and His Creatures: 2.41