19 For the flesh lusts after nothing save through the soul, but the flesh is said to lust against the spirit, when the soul with fleshly lust wrestles against the spirit. This whole are we: and the flesh itself, which on the departure of the soul dies, the lowest part of us is not put away as what we are to flee from, but is laid aside as what we are to receive again, and, after having received it, never again to leave. But “there is sown an animal body, there shall rise again a spiritual body.” Then from that time the flesh will not lust after any thing against the spirit, when as itself also shall be called spiritual, forasmuch as not only without any opposition, but also without any need of bodily aliment, it shall be for ever made subject unto the spirit, to be quickened by Christ.
Therefore these two things, which are now opposed the one to the other within us, since we exist in both, let us pray and endeavor that they may agree. For we ought not to think the one of them an enemy, but the fault, whereby the flesh lusts against the spirit: and this, when healed, will itself cease to exist, and either substance will be safe, and no strife between either. Let us hear the Apostle; “I know,” says he, “that there dwells not in me, that is, in my flesh, any good.” This certainly he says; that the fault of the flesh, in a good thing, is not good; and, when this shall have ceased to exist, it will be flesh, but it will not be now corrupted or faulty flesh.
And yet that this pertains to our nature the same teacher shows, by saying, first, “I know that there dwells not in me,” in order to expound which, he added, “that is, in my flesh, any good.” Therefore he says that his flesh is himself. It is not then itself that is our enemy: and when its faults are resisted, itself is loved, because itself is cared for; “For no one ever hated his own flesh,” as the Apostle himself says. And in another place he says, “So then I myself with the mind serve the Law of God, but with the flesh the Law of sin.”
Let them hear that have ears. “So then I myself;” I with the mind, I with the flesh, but “with the mind I serve the Law of God, but with the flesh the law of sin.” How “with the flesh the law of sin?” was it at all by consenting unto fleshly lust? Far be it! But by having there motions of desires which he would not have, and yet had. But, by not consenting to them, with the mind he served the Law of God, and kept his members from becoming weapons of sins.
Source: On Continence (New Advent)