Augustine realized that the solution [of the origin of evil] was tied to the question: What is evil? The argument above depends on the idea that evil is a thing (note the second premise). But what if evil is not a "thing" in that sense? Then evil did not need creating. If so, our search for the source of evil will take us in a another direction
Augustine approached the problem from a different angle. He asked: Do we have any convincing evidence that a good God exists? If independent evidence leads us to conclude that God exists and is good, then He would be incapable of creating evil. Something else, then, must be its source.
If Augustine's approach is fair, it prompts a pair of syllogisms that lead to a different conclusion. First: 1) All things that God created are good; 2) evil is not good; 3) therefore, evil was not created by God. Second: 1) God created every thing; 2) God did not create evil; 3) therefore, evil is not a thing.
The key to success here, is the truthfulness of two premises. If Augustine can offer evidence through natural theology that God exists as Creator and also that God is good, making everything He created also good, then the conclusion--evil is not a thing--automatically follows.
This is Augustine's strategy. If evil is not a thing, then the case against Christianity stated in the original syllogism is unsound because one of its premises is false. The critical question is: What is evil?
Central to Augustine's idea of goodness (and, consequently, evil) was the notion of being. To Augustine, anything that had being was good. God as the ground of being was perfectly good, along with everything he brought into being. This goodness was a property that came in varying degrees.
With this foundation Augustine was now prepared to answer the key issue: 'Where is evil then, and whence, and how crept it in hither? What is its root, and what its seed? Or hath it no being?' To this Augustine answered: 'Evil has no positive nature; but the loss of good has received the name 'evil.'
Augustine observed that evil always injures, and such injury is a deprivation of good. If there were no deprivation, there would be no injury. Since all things were made with goodness, evil must be the privation of goodness: 'All which is corrupted is deprived of good.'
The diminution of the property of goodness is what's called evil. Good has substantial being; evil does not. It is like a moral hole, a nothingness that results when goodness is removed. Just as a shadow is no more than a 'hole' in light, evil is a hole in goodness.
To say that something is evil, then, is a shorthand way of saying it either lacks goodness, or is a lower order of goodness than what ought to have been. But the question remains: 'Whence and how crept it in hither?'
Augustine observed that evil could not be chosen because there is no evil thing to choose. One can only turn away from the good, that is from a greater good to a lesser good (in Augustine's hierarchy) since all things are good. 'For when the will abandons what is above itself, and turns to what is lower, it becomes evil--not because that is evil to which it turns, but because the turning itself is wicked.'
Evil, then, is the act itself of choosing the lesser good. To Augustine the source of evil is in the free will of persons: 'And I strained to perceive what I now heard, that free-will was the cause of our doing ill.' Evil was a "perversion of the will, turned aside from...God" to lesser things."
"Evil exists because we have free will. God enables humans to freely choose their actions and deeds, and evil inevitably results from these choices."