From a Lenten Sermon given by St. Francis de Sales

[Archer]

The first condition is that we must fast with our whole heart, that is to say, willingly, whole-heartedly, universally and entirely. If I recount to you St. Bernard's words regarding fasting, you will know not only why it is instituted but also how it ought to be kept.

He says that fasting was instituted by Our Lord as a remedy for our mouth, for our gourmandizing and for our gluttony. Since sin entered the world through the mouth, the mouth must do penance by being deprived of foods prohibited and forbidden by the Church, abstaining from them for the space of forty days. But this glorious saint adds that, as it is not our mouth alone which has sinned, but also all our other senses, our fast must be general and entire, that is, all the members of our body must fast. For if we have offended God through the eyes, through the ears, through the tongue, and through our other senses, why should we not make them fast as well? And not only must we make the bodily senses fast, but also the soul's powers and passions—yes, even the understanding, the memory, and the will, since we have sinned through both body and spirit.

How many sins have entered in to the soul through the eyes, as Holy Scripture indicates? [1 Jn 2:16]. That is why they must fast by keeping them lowered and not permitting them to look upon frivolous and unlawful objects; the ears, by depriving them of listening to vain talk which serves only to fill the mind with worldly images; the tongue, in not speaking idle words and those which savor of the world or the things of the world. We ought also to cut off useless thoughts, as well as vain memories and superfluous appetites and desires of our will. In short, we ought to hold in check all those things which keep us from loving or tending to the Sovereign Good. In this way interior fasting accompanies exterior fasting.

(The Sermons of St. Francis de Sales For Lent, Ed. Lewis S. Fiorelli, O.S.F.S., Trans. the Nuns of the Visitation, TAN Books and Publisher, Inc. 1987. 2-3.)