Augustine of Hippo, Against Two Letters of the Pelagians I.19: What It is to Accomplish What is Good

And now does not what follows most plainly show whence he spoke? “For I know that in me, that is, in my flesh, dwelleth no good thing”? For if he had not explained what he said by the addition of “that is, in my flesh,” it might, perchance, be otherwise understood, when he said, “in me.” And therefore he repeats and urges the same thing in another form: “For to will is present with me, but to perform that which is good is not.” For this is to perform that which is good, that a man should not even lust. For the good is incomplete when one lusts, even although a man does not consent to the evil of lust. “For the good that I would,” says he, “I do not; but the evil that I would not, that I do. Now, if I do that I would not, it is no more I that do it, but sin that dwelleth in me.” This he repeated impressively, and as it were to stir up the most slothful from slumber: “I find then that the law,” said he, “is for me wishing to do good, since evil is present with me.” The law, then, is for one who would do good, but evil is present from lust, though he does not consent to this who says, “It is no longer I that do it.”