St. Augustine, bishop of Hippo, was both a theologian of great influence and a philosopher of remarkable originality. He helped shape Christian orthodoxy by identifying the Christian heresies of Pelagianism, Manicheanism, and Donatism, the first two of which have special philosophical interest. Pelagianism, as captured by the maxim philosophers associate with Kant, ‘Ought implies can,’ stakes out a plausible limit on moral responsibility. Augustine’s idea that human beings are obligated to obey the moral law despite the fact that, after the fall of Adam, they have been in a state of depravity in which they can do no good apart from the grace of God, poses a direct challenge to this plausible limit on moral responsibility (see Chapter 29). Augustine also sought to refute Manicheanism, according to which there is a cosmic principle of evil and darkness coeval with the principle of goodness and light. In responding to this attractive way of thinking about the origin of evil in the world, Augustine came up with several responses to the problem of evil, responses that directly influenced medieval discussions of the topic.

In writing no fewer than five detailed commentaries on the creation story in the biblical book of Genesis, Augustine did perhaps as much as any philosopher has done to try to make sense of the idea that God created the world out of nothing. Indeed, in the thirteenth-century debate on whether the world is eternal Augustine’s view of ex nihilo creation became the antipode to the Aristotelian view that the world had no beginning (see Chapter 17).