Pelagius agreed with Augustine that sin cannot be a substance, since God doesn’t create evil. For Pelagius, this meant that sin cannot corrupt or wound or weaken human nature since “how could that which lacks substance have weakened or changed human nature.” Augustine’s response is to quote Scripture (Psalm 40), and to point out that absences - such as the absence of food - can affect our existence.

Rusty Reno points out that Augustine’s argument implies a new Christian ontology: “on the question of sin, Augustine pushes the notion of substance or essence into the background and draws the particular grammar of Christian revelation into the foreground. This shift away from substance or essence as the determinate ground of ‘the real’ is, I would argue, characteristic of the Christian theological tradition as a whole.”

Christianity introduced the category of “insubstantial reality,” a category that busts open all Greek metaphysics.