Chapter I.
On the origin of evil in general.
Having therefore briefly predetermined certain things concerning the Trinity of God and the creature of the world, it now remains to touch briefly upon some matters concerning the corruption of sin. Regarding this, it must be held in summary that sin is not some essence, but a defect and corruption, by which namely measure, species, and order are corrupted in the created will; and on this account the corruption of sin is contrary to the good itself, yet it does not have being except in the good, nor does it draw its origin except from the good, which is indeed the free choice of the will: and this itself is neither supremely evil, since it can will the good, nor supremely good, since it can decline into evil.
Now the reason for understanding the aforesaid is this: because the first principle, since it is a being from itself, not from another, it is necessary that it be a being for the sake of itself, and on this account supremely good, having absolutely no defect. Therefore there neither is nor can be anything that is the first and supremely evil, because the first principle denotes supreme completeness, and the supreme evil denotes the greatest defect. Since therefore the first principle, as supremely being and complete, can fail neither in being nor in operating: it is neither supremely evil nor is it any evil, nor can it in any way originate evil.
Yet because it is omnipotent, it can bring the good from non-being into being, even without the aid of any matter. And this it did when it fashioned the creature to which it gave being, living, understanding, and willing: which indeed, because it was from the supreme good according to the relation of a threefold cause, it was fitting that it should have in its substance and will measure, species, and order. It was therefore born to perform its works from God and according to God and for the sake of God, and this according to the measure, species, and order implanted in it.
But because it was from nothing and defectible, it could fail in acting for the sake of God, so as to do something for the sake of itself, not for the sake of God, and on this account neither from God nor according to God nor for the sake of God: and this is sin, which is destructive of measure, species, and order: which, because it is a defect, does not have an efficient cause but a deficient one, namely the defect of the created will.
But because it is a corruption, and only of the good, and every corruption exists in a corruptible thing: therefore it exists only in the good; and on this account, since the free will corrupts in itself measure, species, and order by failing from the true good, every sin insofar as it is such both is from the will as from its first origin, and is in the will as in its proper subject: which indeed the will does when, by its own defectibility, mutability, and convertibility, having spurned the unfailing and unchangeable good, it clings to a changeable good.
From which it is gathered that "sin is not the desire for evil things, but the abandonment of better things"; and therefore in the appetite of the will it is destructive of measure, species, and order, and consequently "so voluntary that if it is not voluntary, it is not sin."
With these things understood beforehand, the impiety of the Manichaeans, who posit a supremely evil first principle of all evils, manifestly collapses. It is also apparent what the origin of evil is, and what the subject of evil is.