-Philokalia Volume IV"Rejoice, O young man, in your youth; and walk in the ways of your heart, blameless, expelling anger from your heart; and if the spirit of the ruler rises up against you do not desert your place," 'place' meaning your heart.
Similarly our Lord [Jesus Christ] also says, "out of the heart proceed evil thoughts", and "do not be distracted". And again, "straight is the gate and narrow is the way that leads to life". Elsewhere He also says, "Blessed are the poor in spirit"; that is to say, blessed are those who are destitute of every worldly thought. Saint Peter says likewise, "Be watchful, be vigilant, because your adversary, the devil, walks about like a roaring lion, seeking whom he may devour". [...] In short, if you do not guard your intellect you cannot attain purity of heart, and so to be counted worthy to see God. 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. 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 [...] I will tell you about this too, in so far as I can.
Above all else you should strive to acquire three things, and so begin to attain what you seek. The first is freedom from anxiety with respect to everything, whether reasonable or senseless - in other words, you should be dead to everything. Secondly, you should strive to preserve a pure conscience, so that it has nothing to reproach you with. Thirdly, you should be completely detached, so that your thoughts incline towards nothing worldly, not even your own body.
Then, sit down in a quiet cell, in a corner by yourself, and do what I tell you. Close the door, and withdraw your intellect from everything worthless and transient. [...] Restrain the drawing-in of breath through your nostrils, so as not to breathe easily, and search inside yourself with your intellect so as to find the place of the heart, where all the powers of the soul reside.
To start with you will find there darkness and an impenetrable density. Later, when you persist and practice this day and night, you will find, as though miraculously, an unceasing joy. For as soon as the intellect attains the place of the heart, at once it sees things of which is previously knew nothing. It sees the open space within the heart and it beholds itself entirely luminous and full of discrimination. From then on, from whatever side a distractive thought may appear, before it has come to completion and assumed a form, the intellect immidiately drives it away and destroys it with the invocation of Jesus Christ.