Having cleared the ground and indicated in a preliminary way the true character of attentiveness, let us now speak clearly and concisely about its characteristics. True and unerring attentiveness and prayer mean that the intellect keeps watch over the heart while it prays; it should always be on patrol within the heart, and from within — from the depths of the heart — it should offer up its prayers to God. Once it has tasted within the heart that the Lord is bountiful (cf. Ps. 34:8. LXX), then the intellect will have no desire to leave the heart, and it will repeat the words of the Apostle Peter, ‘It is good for us to be here’ (Matt. 17:4). It will keep watch always within the heart, repulsing and expelling all thoughts sown there by the enemy. To those who have no knowledge of this practice it appears extremely harsh and arduous; and indeed it is oppressive and laborious, not only to the uninitiated, but also to those who, although genuinely experienced, have not yet felt the delight to be found in the depths of the heart. But those who have savoured this delight proclaim with St. Paul, ‘Who will separate us from the love of Christ?’ (Rom. 8:35).
Our holy fathers hearkened to the Lord’s words, ‘Out of the heart proceed evil thoughts, murders, adulteries, unchastity, thefts, perjuries, blasphemies; these are the things that defile a man’ (Matt. 15:19-20); and they also hearkened to Him when He enjoins us to cleanse the inside of the cup so that the outside may also be clean (cf. Matt. 23:26). Hence they abandoned all other forms of spiritual labour and concentrated wholly on this one task of guarding the heart, convinced that through this practice they would also possess every other virtue, whereas without it no virtue could be firmly established. Some of the fathers have called this practice stillness of the heart, others attentiveness, others the guarding of the heart, others watchfulness and rebuttal, and others again the investigation of thoughts and the guarding of the intellect. But all of them alike worked the earth of their own heart, and in this way they were fed on the divine manna (cf. Exod. 16:15).
Ecclesiastes is referring to this when he says, ‘Rejoice, O young man, in your youth; and walk in the ways of your heart’ (Eccles. 11:9), blameless, expelling anger from your heart; and ‘if the spirit of the ruler rises up against you, do not desert your place’ (Eccles. 10:4), by ‘place’ meaning the heart. Similarly our Lord also says, ‘Out of the heart proceed evil thoughts’ (Matt. 15:19), and ‘Do not be distracted’ (Luke 12:29). And again, ‘Strait is the gate and narrow is the way that leads to life’ (Matt. 7:14). Elsewhere He also says, ‘Blessed are the poor in spirit’ (Matt. 5:3); that is to say, blessed are those who are destitute of every worldly thought. St. Peter says likewise, ‘Be watchful, be vigilant, because your adversary, the devil, walks about like a roaring lion, seeking whom he may devour’ (1 Pet. 5:8). And St. Paul writes very plainly to the Ephesians about the guarding of the heart, ‘We do not wrestle against flesh and blood’ (Eph. 6:12), and so on. And our holy fathers have also spoken in their writings about guarding the heart, as those who wish can see for themselves by reading what Mark the Ascetic, St. John Klimakos, St. Hesychios the Priest, St. Philotheos of Sinai, St. Isaiah the Solitary and St. Varsanuphios, and the entire book known as The Paradise of the Fathers, have to say about the subject.
In short, if you do not guard your intellect you cannot attain purity of heart, so as to be counted worthy to see God (cf. Matt. 5:18). Without such watchfulness you cannot become poor in spirit, or grieve, or hunger and thirst after righteousness, or be truly merciful, or pure in heart, or a peacemaker, or be persecuted for the sake of justice (cf. Matt. 5:3-10). To speak generally, it is impossible to acquire all the other virtues except through watchfulness. For this reason you must pursue it more diligently than anything else, so as to learn from experience these things, unknown to others, that I am speaking to you about. Now if you would like to learn also about the method of prayer, with God’s help I will tell you about this too, in so far as I can.
Excerpt from the Philokalia: The Three Methods of Prayer attributed to St. Symeon the New Theologian