alm cxxxvii:
"Happy shall he be, that taketh and dasheth the little ones of
Babylon against the rock," [Ps. 137:9] that is, if the heart runs
to the Lord Christ with its evil thoughts while they are yet
young and just beginning; for Christ is a Rock, on which they are
ground to powder and come to naught.

See, here each one will find enough to do with himself, and more
than enough, and will be given many good works to do within
himself. But now no one uses prayer, fasting, watching, labor for
this purpose, but men stop in these works as if they were in
themselves the whole purpose, although they should be arranged so
as to fulfil the work of this Commandment and purify us daily
more and more. Some have also indicated more things which should
be avoided, such as soft beds and clothes, that we should avoid
excessive adornment, and neither associate nor talk with members
of the opposite sex, nor even look upon them, and whatsoever else
may be conducive to chastity. In all these things no one can fix
a definite rule and measure. Each one must watch himself and see
what things are needful to him for chastity, in what quantity and
how long they help him to be chaste, that he may thus choose and
observe them for himself; if he cannot do this, let him for a
time give himself up to be controlled by another, who may hold
him to such observance until he can learn to rule himself. This
was the purpose for which the monastic houses were established of
old, to teach young people discipline and purity.

[Sidenote: Faith as a Help to Chastity]

III. In this work a good strong faith is a great help, more
noticeably so than in almost any other; so that for this reason
also Isaiah xi. says that "faith is a girdle of the reins," [Is.
11:5] that is, a guard of chastity. For he who so lives that he
looks to God for all grace, takes pleasure in spiritual purity;
therefore he can so much more easily resist fleshly impurity: and
in such faith the spirit tells him of a certainty how he shall
avoid evil thoughts and everything that is repugnant to chastity.
For as the faith in divine favor lives without ceasing and works
in all works, so it also does not cease its admonitions in all
things that are pleasing to God or displease Him; as St. John
says in his Epistle: "Ye need not that any man teach you: for the
divine anointing, that is, the Spirit of God, teacheth you of all
things." [1 John 2:27]

Yet we must not despair if we are not soon