Augustine of Hippo, On Marriage and Concupiscence I.32: When Good Will Be Perfectly Done

The apostle then adds these words: “For I know that in me (that is, in my flesh) dwelleth no good thing: for to will is present with me; but how to perfect that which is good I find not.” Now this is said, because a good thing is not then perfected, when there is an absence of evil desires, as evil is perfected when evil desires are obeyed. But when they are present, but are not obeyed, neither evil is performed, since obedience is not yielded to them; nor good, because of their inoperative presence. There is rather an intermediate condition of things: good is effected in some degree, because the evil concupiscence has gained no assent to itself; and in some degree there is a remnant of evil, because the concupiscence is present. This accounts for the apostle’s precise words. He does not say, To do good is not present to him, but “how to perfect it.” For the truth is, one does a good deal of good when he does what the Scripture enjoins, “Go not after thy lusts;” yet he falls short of perfection, in that he fails to keep the great commandment, “Thou shalt not covet.” The law said, “Thou shalt not covet,” in order that, when we find ourselves lying in this diseased state, we might seek the medicine of Grace, and by that commandment know both in what direction our endeavours should aim as we advance in our present mortal condition, and to what a height it is possible to reach in the future immortality. For unless perfection could somewhere be attained, this commandment would never have been given to us.