"At that time also there came into my hands a certain book of Pelagius', in which he defends, with all the argumentative skill he could muster, the nature of man, in opposition to the grace of God whereby the unrighteous is justified and we become Christians. The treatise which contains my reply to him, and in which I defend grace, not indeed as in opposition to nature, but as that which liberates and controls nature, I have entitled On Nature and Grace. In this work sundry short passages, which were quoted by Pelagius as the words of the Roman bishop and martyr, Xystus, were vindicated by myself as if they really were the words of this Sixtus. For this I thought them at the time; but I afterwards discovered, that Sextus the heathen philosopher, and not Xystus the Christian bishop, was their author. This treatise of mine begins with the words: 'The book which you sent me.'"