To Florus 1.71

But enough of this; I will now take up what follows. “In the body of this death, then, a body such as certainly did not exist in paradise before sin, another law in our members fights against the law of our mind. Even when we do not will it, do not consent to it, and do not offer our members for it to accomplish what it desires, it nevertheless dwells in them and troubles the mind that resists and fights back. Even if the conflict itself does not merit condemnation, because it does not carry out injustice, it is still a miserable struggle because it has no peace” (Augustine, On Marriage and Concupiscence 2.6; compare Romans 7:23).

The universal testimony of humanity teaches us that the pleasure of all the senses is natural. That this pleasure and desire existed in paradise before sin is shown by the fact that desire was the route to the offense. After the fruit's beauty excited the eyes, it also stirred up the hope of a pleasant taste (Genesis 3:6). This desire sins when it fails to keep its proper measure, but when it stays within the bounds of what is allowed, it is a natural and innocent feeling. It cannot, I say, have been a fruit of sin when it is shown to have been an occasion of sin, not through its own fault but through the will's. Read my second book on this too; you will find that what we say can convince even your mind.

As for the supposedly clever distinction you made, that the law of sin is indeed in our members but involves sin only when we consent to it, while when we do not consent it merely starts a battle and inflicts misery by disturbing peace, what sensible person would not see the contradiction? If the law of sin, that is, sin and the necessity of sin, is naturally implanted in our members, what good is it not to consent to it, when one must undergo punishment simply because it exists? Or if it is indeed the law of sin but does not sin when I do not consent to it, then the human will has an immeasurable power: if the absurdity may be uttered, it forces sin itself not to sin! But we return to the fact that your statement cannot hold together. If it does not sin, it is not the law of sin. If it is the law of sin, it sins. And if it sins simply by existing, how can one resist it so that sin does not occur, when it cannot be driven away so that sin's activity may cease?