Bodily fasting alone is not enough to bring about perfect self-restraint and true purity; it must be accompanied by contrition of heart, intense prayer to God, frequent meditation on the Scriptures, toil and manual labor. These are able to check the restless impulses of the soul and to recall it from its shameful fantasies. Humility of soul helps more than everything else, however, and without it no one can overcome unchastity or any other sin. In the first place, then, we must take the utmost care to guard the heart from base thoughts, for, according to the Lord, ‘out of the heart proceed evil thoughts, murders, adulteries, unchastity’ and so on (Matt.15:19).
(St John Cassian, On the Eight Vices, Philokalia V1, P. 75 )