To Florus 5.28

Repeated vices and severe judgments testify that it does. It is therefore established that sin exists. We ask what it is: whether it is some body visibly composed of many elements, or some single thing, like one element considered in thought apart from its combination with the others. But it is none of these. What is it, then? An appetite of free will which justice prohibits. Or, to use the earlier definition: the will to do what justice forbids and from which one is free to abstain. Consider, then, whether sin can be found outside the boundaries of this definition, lest what we suppose contained be wandering elsewhere. Let us consult the judge’s justice, so that its testimony too may establish whether every kind of sin has been properly enclosed by these limits.

Does God impute what he knows cannot be avoided? That would be no justice, but the greatest disgrace. Indeed, if this occurred, sins would not be punished but increased. A just judge ordinarily punishes guilt. If guilt invaded the judge himself through the corruption of justice, it would have taken vengeance upon the judge, not been punished. Justice therefore imputes nothing as sin except what one is free to avoid. But nothing can be called free unless, without any unavoidable compulsion of natural forces, it lies within the authority of a will possessing its own freedom. The definition is therefore excellent and complete: sin is the will to do what justice forbids and from which one is free to abstain. With these points resolved, let us ask where it comes from—a question asked in great disorder before these definitions. Where, then, does sin come from? I answer: from the doer’s free will.