Saint Hesychius wrote "On Watchfulness and Holiness" which is included in the Piliokalia.
Watchfulness is a spiritual method which if sedulously practiced over a long period, completely frees us with God's help from impassioned thoughts, impassioned words and evil actions. It leads, in so far as this is possible, to a sure knowledge of the inapprehensible God, and help 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...
Watchfulness is a continual fixing of thought at the entrance to the heart...
St. Hesychius identifies four different kinds of watchfulness.
One type of watchfulness consists in closely observing every mental image or provocation; for only by means of mental image can satan fabricate an evil thought and insinuate this into the spirit in order to lead it astray.
A Second type of watchfulness consists in freeing the heart from all thoughts, keeping it profoundly silent and still, and in praying.
A third type consists in continually and humbly calling upon the Lord Jesus Christ for help.
A fourth type is always to have the thought of death in one's mind.These types of watchfulness, my child, act like doorkeepers and bar entry to evil thoughts...
Philokalia, vol 1, p. 162-165
