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 find some people who think that to fast well during the holy season of Lent it is enough to abstain from eating some prohibited food. But this thought is too gross to enter into the hearts of religious, for it is to you I speak, as well as persons dedicated to Our Lord. 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.