Cassian, the great teacher that he is, makes it clear that both internal and external disciplines are needed in the spiritual life and the quest for purity of heart. Above all, humility stands above all the rest because it leads us to distrust the self and place ourselves completely in the care of the "Doctor of our souls."
Bodily fasting alone is not enough to bring about 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 anything 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, murder, adulteries, unchastity, and so on” (Matthew 15:19).
The Doctor of our souls has also placed the remedy in the hidden regions of the soul, recognizing that the cause of our sickness lies there when he says: “Whoever looks at a woman with lust has already committed adultery with her in his heart” (Matthew 5:28). He seeks to correct not so much our inquisitive and unchaste eyes as the soul that has its seat within and makes bad use of the eyes that God gave it for good purposes. That is why the book of Proverbs in its wisdom does not say, “Guard your eyes with all diligence” but “Guard your heart with all diligence” (Proverbs 4:23), imposing the remedy of diligence in the first instance upon that which makes use of the eyes for whatever purpose it desires.
St. John Cassian
I, On the Eight Vices”