Chapter 32 — Part III: The Decalogue — The Ninth and Tenth Commandments
Section 18. Another reason why this sort of vicious desire is condemned is that it has for its object that which belongs to another, such as a house, maid-servant, field, wife, ox, ass and many other things, all of which the law of God forbids us to covet, simply because they belong to another. The desire of such things, when consented to, is criminal, and is numbered among the most grievous sins. For sin is committed the moment the soul, yielding to the impulse of corrupt desires, is pleased with evil things, and either consents to, or does not resist them, as St. James, pointing out the beginning and progress of sin, teaches when he says: Every man is tempted by his own concupiscence, being drawn away and allured; then, when concupiscence hath conceived, it bringeth forth sin; but sin, when it is completed, begetteth death.