Romans 7:19

Paul repeats this often in order to make it clear.

In fact, I would not have known sin if the law had not said: You shall not covet. And further: Without the law, sin is dead. For what benefit is it to me to know what I cannot avoid? What benefit is it to me to know that the law of my flesh opposes me?

For there is a certain weakness of this kind in those who receive the beginnings of conversion: that although one may wish to do at once everything that is good, the effect does not immediately follow the will. For a man, deliberating within himself, resolves (for example) not to grow angry, and he settles this in his will; but because by long use and lasting habit the vice of wrath has held dominion in him, it resists even his will and purpose, and, taking the path customary to it, the force of rage bursts out. The vice of lust acts in like manner. By an equal disease of habit a lie too creeps in, fear frightens him off; and in each of these that man who is now receiving the beginnings of conversion fittingly says that "to will is present with me, but to accomplish the good I find not. For the good that I will, this I do not; but the evil that I hate, that I do." But if the will holds the judgment of what is good, while the habit of carnal vices — which has been called the law of the flesh or the law of the members — resists and creeps in, then from the fact that I bear the will of the good, though I do evils, yet because I do them unwillingly, "I consent unto the law of God, that it is good." For I hate evils, and I desire to do the goods which the law commands. But if against my will I do evils, overcome by the habit of vices and by the practice of sinning, "it is no more I" — that is, the inner man — "who do the good I will; but sin that dwelleth in me." For he has named the habit of sinning "sin." "I find then a law, that, when I would do good, evil is present with me. For I delight in the law of God after the inward man." And in these words, as we have often noted above, we seem not to be fully unfolded in the expression, but to be hindered either by the abundance of the conjunctions or by their peculiarities. […]

Therefore it is in our power to will not to think these things; but not to bring it about that they shall pass away, so as not to come into the mind again; for this does not lie in our power, as I said; which is the meaning of that statement, "The good that I would, I do not; "for I do not will to think the things which injure me; for this good is altogether innocent. But "the good that I would, I do not; but the evil which I would not, that I do; "not willing to think, and yet thinking what I do not will. And consider whether it was not for these very things that David entreated God, grieving that he thought of those things which he did not will: "O cleanse Thou me from my secret faults. Keep Thy servant also from presumptuous sins, lest they get the dominion over me; so shall I be undefiled, and innocent from the great offence." And the apostle too, in another place: "Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ."

Do you see, how he acquits the essence of the soul, as well as the essence of the flesh, from accusation, and removes it entirely to sinful actions? For if the soul willeth not the evil, it is cleared: and if he does not work it himself, the body too is set free, and the whole may be charged upon the evil moral choice. For willing is indeed natural, and is from God: but willing on this wise is our own, and from our own mind.