Besides the objection mentioned in the last post, St. Thomas brings up another objection to the existence of God:

Objection 1. It seems that God does not exist; because if one of two contraries be infinite, the other would be altogether destroyed. But the word “God” means that He is infinite goodness. If, therefore, God existed, there would be no evil discoverable; but there is evil in the world. Therefore God does not exist.

He replies to the objection,

As Augustine says (Enchiridion xi): “Since God is the highest good, He would not allow any evil to exist in His works, unless His omnipotence and goodness were such as to bring good even out of evil.” This is part of the infinite goodness of God, that He should allow evil to exist, and out of it produce good.

Does good in fact come from evil, and if so, how does this happen?

Some people are born without the ability to feel physical pain. While someone first hearing about this might think it sounds like a good thing, it is in fact a very bad thing, as noted in the linked post. Children suffering from this condition often bite off parts of their tongue, or fail to notice sprains or broken bones, thus causing greater injury to themselves, and so on.

And after a moment’s thought, this is not so strange. Pain, taken as knowledge of an injury, is not a bad thing, but a good thing; it is the injury which is bad, as well as secondary effects such as distraction from other tasks and so on.

And here we see the primary path, although not the only path, by which good comes from evil, at least in human things. Evil is an indirect cause of the knowledge of evil, and the knowledge of evil is good, and a cause of goodness. In this way a person can benefit even from his own faults and vices, insofar as he learns from them and goes on to do better.

This can happen even with very great evils. Thus for example in the post here, Cameron Harwick discusses how the use of nuclear weapons by the United States, a very great evil, might bear significant responsibility for the peace (such as it was) that followed. Or in other words, without that specific use of nuclear weapons, there might have been a nuclear war between the United States and Russia, which would have been much worse.

None of this is accidental, but in fact tends to happen in a very systematic way, basically for reasons which we discussed some days ago. All things and all people strive for the good, and the knowledge of evil is just one of the things that contributes to their efforts.

Now someone might suppose that the explanation here is too human. Does it not take away the glory from God? Should we not say instead that God brings good out of evil in mysterious ways that are beyond our comprehension?

Now there are surely many things which are beyond our comprehension. But the attitude in this objection is simply the zeal for God which is not according to knowledge. God does not make things in such a way that he merely happens to bring good out of evil, but rather he gives the things themselves such an order and such a nature that it is natural to them to bring good out of evil. And such a creation, which possesses this ability in an intrinsic way, is better and more ordered than a theoretical creation that lacked that ability.