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