Augustine of Hippo
Catena Aurea · Matthäusevangelium 5
Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
(Serm. in Mont. i. 12.) For there are three things which make up a sin; suggestion either through the memory, or the present sense; if the thought of the pleasure of indulgence follows, that is an unlawful thought, and to be restrained; if you consent then, the sin is complete. For prior to the first consent, the pleasure is either none or very slight, the consenting to which makes the sin. But if consent proceeds on into overt act, then desire seems to be satiated and quenched. And when suggestion is again repeated, the contemplated pleasure is greater, which previous to habit formed was but small, but now more difficult to overcome.