(CG 3/6) That Evil is a Privation of Good (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

Evil is nothing else than a privation of that which a thing is naturally apt to have and ought to have. But a privation is not an essence, but a negation in a substance.

5. Every essence is natural to some thing. If the essence ranks as a substance, it is the very nature of the thing. If it ranks as an accident, it must be caused by the principles of some substance, and thus will be natural to that substance, though perhaps not natural to some other substance. But what is in itself evil cannot be natural to anything: for the essence of evil is privation of that which is naturally apt to be in a thing and is due to it. Evil then, being a privation of what is natural, cannot be natural to anything. Hence whatever is naturally in a thing is good, and the want of it an evil. No essence then is in itself evil.

6. Whatever has any essence is either itself a form or has a form, for by form everything is assorted in some genus or species. But form, as such, has a character of goodness, being the principle of action and the end which every maker intends, and the actuality whereby every subject of form is perfected. Whatever therefore has any essence, as such, is good.

7. Being is divided into actuality and potentiality. Actuality, as such, is good, because everything is perfected by that whereby it actually is. Potentiality too is something good: for potentiality tends to actuality, and is proportionate to actuality, not contrary to it; and is of the same genus with actuality; and privation does not attach to it except accidentally. Everything therefore that is, in whatsoever way it is, in so far as it is a being, is good.

8. All being, howsoever it be, is from God (B. II, Chap. VI). But God is perfect goodness (B. I, Chap. XLI). Since then evil cannot be the effect of goodness, it is impossible for any being, as being, to be evil.

Hence it is said: God saw all things that he had made, and they were very good (Gen. i, 31): He made all things good in his own time (Eccles. iii, 11): Every creature of God is good (1 Tim. iv, 4).