Taken from Medulla S. Thomae Aquinatis which arranges various short texts from St. Thomas Aquinas' corpus based on the liturgical year. I will be posting the day's meditation each day and will be bringing it into print once I'm through it.
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Thursday after Ash Wednesday
I. Fasting is assumed principally for three reasons:
First, in order to bridle the lusts of the flesh, wherefore the Apostle says (2 Cor 6:5, 6): In fasting, in chastity, since fasting is the guardian of chastity. For, according to Jerome Venus is cold when Ceres and Bacchus are not there, that is to say, lust is cooled by abstinence in meat and drink.
Second, we have recourse to fasting in order that the mind may arise more freely to the contemplation of heavenly things: hence it is related (Dan 10) of Daniel that he received a revelation from God after fasting for three weeks.
Third, in order to satisfy for sins: wherefore it is written (Joel 2:12): Be converted to Me with all your heart, in fasting and in weeping and in mourning. The same is declared by Augustine in a sermon (De Orat. et Jejun.): Fasting cleanses the soul, raises the mind, subjects one’s flesh to the spirit, renders the heart contrite and humble, scatters the clouds of concupiscence, quenches the fire of lust, kindles the true light of chastity.
II. Fasting is a matter of precept. For, fasting is useful as atoning for and preventing sin, and as raising the mind to spiritual things. And everyone is bound by the natural dictate of reason to practice fasting as far as it is necessary for these purposes. Wherefore fasting in general is a matter of precept of the natural law, while the fixing of the time and manner of fasting as becoming and profitable to the Christian people, is a matter of precept of positive law established by ecclesiastical authority: the latter is the Church fast, the former is the fast prescribed by nature.
III. The times for the fasts are fittingly ascribed. For, fasting is directed to two things, the deletion of sin, and the raising of the mind to heavenly things. Wherefore fasting ought to be appointed specially for those times, when it behooves man to be cleansed from sin, and the minds of the faithful to be raised to God by devotion.
These things are particularly requisite before the feast of Easter, when sins are loosed by baptism, which is solemnly conferred at the Easter Vigil, on which day our Lord’s burial is commemorated, because we are buried together with Christ by baptism unto death. Moreover at the Easter festival the mind of man ought to be devoutly raised to the glory of eternity, which Christ restored by rising from the dead, and so the Church ordered a fast to be observed immediately before the Paschal feast; and for the same reason, on the eve of the chief festivals, because it is then that one ought to make ready to keep the coming feast devoutly.
(Selections from ST.II-II.Q147.A1, 3, 5)