To Florus 5.43

For what is evil, that is, sin? The will to pursue what justice forbids and from which one is free to abstain. What is will itself? A movement of the mind under no compulsion. If sin therefore arose from will, and will from a movement of the mind under no compulsion, neither the condition of nothing nor that of darkness caused this movement to exist. It is compelled by no one precisely so that it can exist under no compulsion. Consequently, there is no natural sin and no original sin. These two names indicate one thing: sin that is not voluntary. But truth has ruled that sin cannot exist unless voluntary. Therefore whoever calls evil what is admittedly innate does not prove that sin exists in nature; he shows himself guilty through a perverse judgment. Here is the answer to what you thought could not be answered. The question you regard as invincible was altogether clever foolishness.