At the beginning of the third book, the dialogue has reached the point where several issues have been resolved, but one thing still remains to discuss in detail: the source of evil. Evodius confronts Augustine: “It has been demonstrated to my satisfaction that free will is to be numbered among good things, and indeed not among the least of them, and therefore that it was given to us by God, who acted rightly in giving it. So now, if you think that this is a good time, I would like you to explain the source of the movement by which the will turns away from the common and unchangeable good toward its own good, or the good of others, or lower goods, all of which are changeable” (70). Evodius is content to say that human beings have free will, and that free will is a good given to human nature by God, but he is still unsure of how it is that this good faculty can be put to wicked use.

Augustine continues with a thought experiment and compares the will to an artifact that is only moved from the outside and by necessity.