Augustine, the origin of evil, and the mystery of free will

The question of why humanity first chose to sin is an extension to the problem of evil to which the free-will defence does not easily apply. In De libero arbitrio and elsewhere Augustine argues that as an instance of evil, the fall is necessarily inexplicable. In this article, I identify the problems with this response and attempt to construct an alternative based on Peter van Inwagen's free will 'mysterianism'. I will argue that the origin of evil is inexplicable not because it is an instance of evil, but because it is an instance of free will.