Epistle to Autolycus: Book III 13Of Chastity.

And concerning chastity, the holy word teaches us not only not to sin in act, but not even in thought, not even in the heart to think of any evil, nor look on another man's wife with our eyes to lust after her. Solomon, accordingly, who was a king and a prophet, said: "Let thine eyes look right on, and let thine eyelids look straight before thee: make straight paths for your feet." And the voice of the Gospel teaches still more urgently concerning chastity, saying: "Whosoever looketh on a woman who is not his own wife, to lust after her, hath committed adultery with her already in his heart." "And he that marrieth," says [the Gospel], "her that is divorced from her husband, committeth adultery; and whosoever putteth away his wife, saving for the cause of fornication, causeth her to commit adultery." Because Solomon says: "Can a man take fire in his bosom, and his clothes not be burned? Or can one walk upon hot coals, and his feet not be burned? So he that goeth in to a married woman shall not be innocent."