While we were still delayed at Rome [due to bad weather], we wanted to inquire through argument into the origin of evil. We conducted our discussion in such a way that, if we could, our considered and detailed reasoning would lead us to understand what we believed about this topic by divine authority – to the extent that we could do so by examination, with God's assistance. And since we agreed after careful reasoning that the sole origin of evil is the free choice of the will, the three books which our discussion produced were called On the Free Choice of the Will. After I was ordained a priest at Hippo Regius, I finished off the second and third books as well as I could at the time.

So many issues were examined in these books that I postponed some incidental questions – which either I could not untangle or which demanded a lengthy discussion – so that when it was not clear what came closer to the truth, our reasoning then would nonetheless draw the conclusion from each side (or from all the sides) of these selfsame incidental questions, in order that whichever of them may be true, we could believe, or even prove, that God ought to be praised.

The discussion was undertaken on account of those who deny that the origin of evil lies in the free choice of the will, and who contend that, if this is so, God as the Creator of all natures ought to be blamed; as a result, they want to introduce some unchangeable nature of evil that is co-eternal with God in accordance with their irreligious error (for they are Manichaeans).