Chapter 26 — Part III — The Ten Commandments
Section 603. Q. Why are we forbidden not only evil deeds, but also evil wishes and thoughts? A. First, because when the soul entertains any evil wishes or thoughts, it is already impure in God's sight, and unworthy of him; as Solomon says: The unjust thought is an abomination to the Lord. Prov. xv. 26. And therefore we must needs cleanse ourselves also from these inward impurities also, as the Apostle teaches: Let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of the Lord. 2 Cor. vii. 1. Secondly, because, to prevent sinful acts, it is necessary to crush sinful wishes and thoughts, from which, as from seeds, such actions spring; as it is said: For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies. Matt. xv. 19. Every man is tempted, when he is drawn away of his own lust, and enticed. Then, when lust hath conceived, it bringeth forth sin; and sin, when it is finished, bringeth forth death. James i. 14, 15.