Chapter 32 — Part III: The Decalogue — The Ninth and Tenth Commandments
Section 16. What Sort Of Concupiscence Is Here Prohibited — It is not, then, the mere power of desire, which can move either to a good or a bad object that is prohibited by these Commandments; it is the indulgence of evil desire, which is called the concupiscence of the flesh, and the fuel of sin, and which when accompanied by the consent of the will, is always sinful. Therefore only that covetousness is forbidden which the Apostle calls the concupiscence of the flesh, that is to say, those motions of desire which are contrary to the dictates of reason and outstep the limits prescribed by God.