II. The teaching of the four yearly fasts is that spiritual self-restraint is as necessary as corporeal
This profitable observance, dearly beloved, is especially laid down for the fasts of the Church, which, in accordance with the Holy Spirit's teaching, are so distributed over the whole year that the law of abstinence may be kept before us at all times. Accordingly we keep the spring fast in Lent, the summer fast at Whitsuntide, the autumn fast in the seventh month, and the winter fast in this which is the tenth month, knowing that there is nothing unconnected with the Divine commands, and that all the elements serve the Word of God to our instruction, so that from the very hinges on which the world turns, as if by four gospels we learn unceasingly what to preach and what to do.
For, when the prophet says, “The heavens declare the glory of God, and the firmament shows His handiwork: day unto day utters speech, and night shows knowledge,” what is there by which the Truth does not speak to us? By day and by night His voices are heard, and the beauty of the things made by the workmanship of the One God ceases not to instil the teachings of Reason into our hearts' ears, so that “the invisible things of God may be perceived and seen through the things which are made,” and men may serve the Creator of all, not His creatures. Since therefore all vices are destroyed by self-restraint, and whatever avarice thirsts for, pride strives for, luxury lusts after, is overcome by the solid force of this virtue, who can fail to understand the aid which is given us by fastings?
For therein we are bidden to restrain ourselves, not only in food, but also in all carnal desires. Otherwise it is lost labour to endure hunger and yet not put away wrong wishes; to afflict oneself by curtailing food, and yet not to flee from sinful thoughts. That is a carnal, not a spiritual fast, where the body only is stinted, and those things persisted in, which are more harmful than all delights. What profit is it to the soul to act outwardly as mistress and inwardly to be a captive and a slave, to issue orders to the limbs and to lose the right to her own liberty?
That soul for the most part (and deservedly) meets with rebellion in her servant, which does not pay to God the service that is due. When the body therefore fasts from food, let the mind fast from vices, and pass judgment upon all earthly cares and desires according to the law of its King
Source: Sermons (New Advent)