He demands such great uprightness with good reason. For who can deny that it is right for all the powers of the soul to be possessed with love? But if any soul wander from the goal of love, who will not admit that it is diseased? Now how does it happen that desires hurtful to your brother enter your heart, unless it is that you disregard him and strive for yourself alone? For if your whole heart were steeped in love, not one particle of it would lie open to such imaginings. The heart, then, in so far as it harbors covetousness, must be empty of love.
Someone will object that fantasies, flitting aimlessly about the mind and then vanishing, cannot be condemned as instances of covetousness, whose seat is in the heart. I reply: here it is a question of fantasies of a kind which, while they occupy our minds, at the same time bite and strike our hearts with greed, for nothing desirable ever comes into our mind without our heart leaping with excitement. God therefore commands a wonderful ardor of love, which he does not allow one particle of covetousness to hinder. He requires a marvelously tempered heart, and does not permit the tiniest pinprick to urge it against the law of love. Do you think my view lacks authority? It was Augustine who first opened the way for me to understand this commandment. It was the Lord’s plan to forbid all evil desire. Nevertheless, by way of example, he has put forward those objects whose false image of delight most frequently captivates us. Thus he leaves nothing to our desire when he deprives it of those very things which prompt it to rave and revel. (Institutes, 2.8.50)