Above all this, the strongest defence is prayer and the Word of God; namely, that when evil lust stirs, a man flee to prayer, call upon God's mercy and help, read and meditate on the Gospel, and in it consider Christ's sufferings. Thus says Psalm cxxxvii: "Happy shall he be, that taketh and dasheth the little ones of Babylon against the rock," 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. 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," 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."