Chapter 29 — Part III: The Decalogue — The Sixth Commandment
Section 9. Again it is certain that he who abstains not from other sins against chastity, will easily fall into the crime of adultery. By the prohibition of adultery, therefore, we at once see that every sort of immodesty and impurity by which the body is defiled is prohibited. Nay, that every inward thought against chastity is forbidden by this Commandment is clear, as well from the very force of the law, which is evidently spiritual, as also from these words of Christ the Lord: You have heard that it was said to them of old: "Thou shalt not commit adultery." But I say to you, that whosoever shall look on a woman to lust after her, hath already committed adultery with her in his heart.