To Florus 6.17

See, by contrast, how real our struggle is against you and Mani, whose downfall always entangles you, and how swift our triumph over him. We immediately draw the plow of our first definition around everything he vomited out to defame God’s work. We force him to explain what he thinks sin is. It plainly is nothing other than a will seeking what justice forbids and from which it is free to abstain. Once this is established, all the thornbushes of objection that pricked bodily existence are, as the writer says, shown torn out by the roots. [Literary echo: Cicero, On Ends, Book 2.]

As for the feeling of fear and the sensation of pain, through which he thought a storm arose to shipwreck humans, they are shown not merely to compel no evil, but, when checked by a good will, to assist and carry forward justice. Who would fear judgment without fear’s warning? Who would benefit from repentant groans without the cleansing of pain and inward distress? Finally, what power would a judge’s severity have unless it punished voluntary sins by the suffering of inflicted torment?

All these things testify that sin is nothing other than a free will despising justice’s commands. Justice can stand only by reckoning as sin what it knows could freely have been avoided. Consequently, no law can count natural things as faults, and no one takes on guilt for a crime he did not himself commit while able to avoid it. This argument’s power destroys both Mani and the advocate of inherited sin, who, with all the eyes of understanding gouged out, tries to attach an act of will to reproductive seed.