The term “free will” is not itself biblical, but derives from Greek philosophical movements, especially Stoicism. It was introduced into western Christianity by the second-century theologian Tertullian. Augustine retained the term, but attempted to restore a more Pauline meaning to it by emphasising the limitations placed upon the human free will by sin.

Augustine’s basic ideas can be summarised as follows. First, natural human freedom is affirmed: we do not do things out of any necessity, but as a matter of freedom. Second, Augustine argues that human free will has been weakened and incapacitated – but not eliminated or destroyed – through sin. In order for that free will to be restored and healed, it requires the operation of divine grace. Free will really does exist; it is, however, distorted by sin.

In order to explain this point, Augustine deploys a significant analogy. Consider a pair of scales, with two balance pans. One balance pan represents good, and the other evil. If the pans are properly balanced, the arguments in favour of doing good or doing evil could be weighed, and a proper conclusion drawn.

The parallel with the human free will is obvious: we weigh up the arguments in favour of doing good and evil, and act accordingly.

But what, asks Augustine, if the balance pans are loaded? What happens if someone puts several heavy weights in the balance pan on the side of evil? The scales will still work, but they are seriously biased toward making an evil decision.

Augustine argues that this is exactly what has happened to humanity through sin. The human free will is biased toward evil. It really exists, and really can make decisions – just as the loaded scales still work. But instead of giving a balanced judgment, a serious bias exists toward evil. Using this and related analogies Augustine argues that the human free will really exists in sinners, but that it is compromised by sin.