It leads, in so far as this is possible, to a sure knowledge of the inapprehensible God, and helps us to penetrate the divine and hidden mysteries.
It enables us to fulfill every divine commandment in the Old and New Testaments and bestows upon us every blessing of the age to come.
It is, in the true sense, purity of heart, a state blessed by Christ when He says: ’Blessed are the pure in heart, for they shall see God’(Matt. 5:8);
[…] The great lawgiver Moses – or, rather, the Holy Spirit – indicates the pure, comprehensive and ennobling character of this virtue, and teaches us how to acquire and perfect it, when he says: ‘Be attentive to yourself, lest there arise in your heart a secret thing which is an iniquity’ (Deut.15:9 [LXX]).
Here the phrase ‘a secret thing’ refers to the first appearance of an evil thought.
This the Fathers call a provocation introduced into the heart by the devil.
As soon as this thought appears in our intellect, our own thoughts chase after it and enter into impassioned intercourse with it.
Watchfulness is a way embracing every virtue, every commandment.
It is the heart’s stillness and, when free from mental images, it is the guarding of the intellect.
Just as a man blind from birth does not see the sun’s light, so one who fails to pursue watchfulness does not see the rich radiance of divine grace.
[…] Attentiveness is the heart’s stillness, unbroken by any thought. In this stillness the heart breathes and invokes, endlessly and without ceasing, only Jesus Christ who is the Son of God and Himself God.
It confesses Him who alone has power to forgive our sins, and with His aid it courageously faces its enemies.
Through this invocation enfolded continually in Christ, who secretly divines all hearts, the soul does everything it can to keep its sweetness and its inner struggle hidden from men, so that the devil, coming upon it surreptitiously, does not lead it into evil and destroy its precious work.
Watchfulness is a continual fixing and. halting of thought at the entrance to the heart.
In this way predatory and murderous thoughts are marked down as they approach and what they say and do is noted; and we can see in what specious and delusive form the demons are trying to deceive the intellect.
If we are conscientious in this, we can gain much experience and knowledge of spiritual warfare.
Hesychios the Priest (?6th-9th century): On Watchfulness and Holiness chs 1-6, Text from G.E.H. Palmer, Philip Sherrard, and Kallistos Ware (trans. and eds.) The Philokalia: The Complete Text, vol. I (Faber & Faber, London & Boston: 1979), pp. 162-163.