…Therefore our holy fathers, harkening to the Lord Who said: ‘For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies’ and: ‘ These are the things which defile a man’ (Matt. xv. 19-20), hearing also that in another place of the Gospels we are instructed to ‘cleanse first that which is within the cup and platter, that the outside of them may be clean also’ (Matt. xxiii. 26), have renounced all other spiritual work and concentrated wholly on this one doing, that is on guarding the heart, convinced that, through this practice, they would easily attain every other virtue, whereas without it not a single virtue can be firmly established. Some of the fathers called this doing, silence of the heart; others called it attention; yet others—sobriety and opposition (to thoughts), while others called it examining thoughts and guarding the mind. They all practiced it pre-eminently and, through it, received Divine gifts. Ecclesiastes means the same thing when he says: ‘Rejoice, O young man, in thy youth…and walk in the ways of thine heart’ (Eccles. xi. 9) in purity, withdrawing the heart from evil thoughts. In another place he speaks of the same: ‘If the spirit of the ruler wise up against thee, leave not thy place’ (Eccles. x. 4)—by place, meaning heart. The Lord too tells us in the Gospels: ‘Neither be ye of doubtful mind’ (Luke xii. 29)—do not dart about like meteorites, do not rush hither and thither with your mid. And again in another place He says: ‘Blessed are the poor in spirit’ (Matt. v. 3), that is, blessed are those who have no attachment to the world in their heart, but are destitute of worldly thought. All the holy fathers wrote much about this. Those who wish may read their writings and see what St. Mark the Wrestler has written, or St. John of the Ladder, Hesychius of Jerusalem, Philotheus of Sinai, Abba Isaiah, Barsanuphius the Great and many others.
In a word, he who does not have attention in himself and does not guard his mind, cannot become pure in heart and so cannot see God. He who does not have attention in himself cannot be poor in spirit, cannot weep and be contrite, nor be gentle and meek, nor hunger and thirst after righteousness, nor be merciful, nor a peacemaker, nor suffer persecution for righteousness’ sake. Speaking generally, it is impossible to acquire virtue in any other way, except through this kind of attention. Therefore you should try to gain this more than anything else so as to learn what I tell you in your own experience. If you wish also to learn how it should be done, I will tell you of this.