On Two Souls, Against the Manichaeans

Some one will say: What assistance would this have furnished you against the Manichæans? Wait a moment; permit me first also to define sin, which, every mind reads divinely written in itself, cannot exist apart from will. Sin therefore is the will to retain and follow after what justice forbids, and from which it is free to abstain. Although if it be not free, it is not will. But I have preferred to define more roughly than precisely. Should I not also have carefully examined those obscure books, whence I might have learned that no one is worthy of blame or punishment who either wills what justice does not prohibit him from willing, or does not do what he is not able to do?

Do not shepherds on mountains, poets in theatres, unlearned in social intercourse, learned in libraries, masters in schools, priests in consecrated places, and the human race throughout the whole world, sing out these things? But if no one is worthy of blame and condemnation, who either does not act against the prohibition of justice, or who does not do what he cannot do, yet every sin is blameworthy and condemnable, who doubts then that it is sin, when willing is unjust, and not willing is free. And hence that definition is both true and easy to understand, and not only now but then also could have been spoken by me: Sin is the will of retaining or of obtaining, what justice forbids, and whence it is free to abstain?