In book 12, sections 6-9, Augustine makes an argument that the cause of wills turning to evil is that they have turned away from good, not that the wills themselves are evil. 12:9 – “And the will is made evil by nothing else than defection from God – a defection of which the cause, too, is certainly deficient.” Later: “For good is not the cause of evil, but a defection from good is.”
From his reasoning, I would agree. Let’s take Genesis 3 as an explanation for the cause of evil in men. Man was perfectly good and there was no evil in him in the beginning. However, his heart longed to be like God, to make himself a god, and his will turned to a lesser good (i.e. himself). Pride, as they say, is the origin of all sin. He became proud: seeking to exalt himself and be like God. That is the cause of all evil in the world: our wills exalting a lesser good over the ultimate good.
You could even argue that God didn’t really need to put the tree there. It was just a physical representation of a deeper truth: the option to pursue the ultimate good or to exalt oneself as God. Even if God hadn’t placed the tree there, that choice would have come out in some other way. The tree made it obvious, but the choice was always the same: exalt yourself or exalt God. Free will (the way Augustine says “a will,” we use the word “free will” today, I think) would have been there with or without the tree.
So, before the fall, Adam had a free will and it was good. It did not become evil until he ate the fruit. He ate the fruit in pride and which turned his will to evil. But why did God create wills? For His glory. To create a metanarrative of redemptive history. He knew what direction those wills would go, but so that He could redeem them and display His mercy and patience, He allowed the whole race to go astray so that He could redeem His chosen people. Wills serving God are more glorious things than non-wills serving God.
But going back further, what about Satan? He and his angels were the first wills to become evil, and he even played a part in turning Adam’s will to evil. WHOA! According to Exekiel 28:13, Satan was in Eden AS a perfect being. i.e. he fell after the creation, not before.
Anyways, according to verse 17, Satan’s “heart became proud on account of your beauty, and you corrupted your wisdom because of your splendor. So I threw you to the earth…” Satan corrupted himself through his pride (according to NIV… not sure what it’s like in the original Hebrew). Pride was his sin from the beginning. According to Isaiah 14:13-14 as well, Satan tried to ascend and be like the most high, and that’s what caused him to be brought low.
So, from the beginning, pride (self-exaltation) was the sin of Satan, and it caused him to be brought low. He therefore tried to bring down mankind as well, which he did at the garden of Eden. I truly see why they say that pride is the root of all sin.
Question: Why did God create the tree of the knowledge of good and evil?
Response: Could Adam have fallen without the tree? Yes. He had a will, so with or without the tree, he would have had the choice to follow God or exalt himself. But God gave him the tree, I believe, to make the choice an obvious one, rather than a subtle one.
Question: OK, so why did God give Adam a will in the first place? With or without the tree, that’s the real question?
Response: Because God saw greater good and glory in redeeming a fallen creation and wooing spoiled wills back to himself than He saw in a creation without wills that could not fall.