Chapter 32 — Part III: The Decalogue — The Ninth and Tenth Commandments
Section 3. Necessity Of Promulgating These Two Commandments — The promulgation of these two Commandments was necessary for two reasons. The first is to explain the sixth and seventh Commandments. Reason alone shows that to prohibit adultery is also to prohibit the desire of another man's wife, because, were the desire lawful, its indulgence must be so too; nevertheless, many of the Jews, blinded by sin, could not be induced to believe that such desires were prohibited by God. Nay, even after the Law had been promulgated and become known, many who professed themselves its interpreters, continued in the same error, as we learn from these words of our Lord recorded in St. Matthew: You have heard that it was said to them of old: "Thou shalt not commit adultery," but I say to you, etc.