From one of his Sermons for Ash Wednesday, St. Francis de Sales on fasting:
To treat of fasting and of what is required to fast well, we must, at the start, understand that of itself fasting is not a virtue. The good and the bad, as well as Christians and pagans, observe it. The ancient philosophers observed it and recommended it. They were not virtuous for that reason, nor did they practice virtue in fasting. Oh, no, fasting is a virtue only when it is accompanied by conditions which render it pleasing to God. Thus it happens that it profits some and not others, because it is not undertaken by all in the same manner.
...We know very well that it is not enough to fast exteriorly if we do not also fast interiorly and if we do not accompany the fast of the body with that of the spirit.
That is why our Divine Master, who instituted the fast, greatly desired in His Sermon on the Mount to teach His Apostles how it must be practiced (Matt. 6: 16-18), which is a matter of great profit and utility....He knew that to draw strength and efficacy from fasting, something more than abstinence from prohibited food is necessary. Thus He instructed them and, consequently, disposed them to gather the fruits proper to fasting. Among many others are these four: fasting fortifies the spirit, mortifying the flesh and its sensuality; it raises the spirit to God; it fights concupiscence and gives power to conquer and deaden its passions; in short, it disposes the heart to seek to please only God with great purity of heart.
It will be very helpful to state clearly what must be done to fast well these forty days.... Now, among all the conditions required for fasting well, I will select three principal ones and speak familiarly about them.
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 goods 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 the 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 into 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 to cut off useless thoughts, as well as vain memories and superfluous appetites and desires of our will. In short, we ought to check all those things which keep us from loving or tending to the Sovereign Good. In this way interior fasting accompanies exterior fasting.