The nineteenth century Russian story, The Candid Narrations of a Pilgrim to His Spiritual Father, tells the story of a poor man with a withered arm who reads St. Paul's instruction to pray without ceasing. Thessalonians 5:17. He wonders how a person can do this. He seeks advice from learned spiritual leaders, who tell him it means prayer of the mind. But they can't tell him how to do it.
He eventually meets a monk, who instructs him with the Philokalia. The monk emphasizes the primacy of prayer; he also stresses the need to calm the mind: “the heavenly light of incessant inner prayer . . . is attained in poverty of spirit, in active experience and in simplicity of heart.” “Poverty in spirit” and “simplicity of heart” are parallel virtues of a calm mind. The calm mind is a simple heart that is not scattered among a multitude of concerns. Poverty of spirit is a simple soul that is not distracted by sundry earthly concerns.
The pilgrim finds a quiet hut and begins to practice the Jesus Prayer. All went well, but soon he began to feel tired, lazy and bored. “Overcome by drowsiness, I was often distracted by all kinds of thoughts that came upon me like a cloud.” He consults the monk, who recognizes the efforts of the “kingdom of darkness.” That kingdom, the monk explains, hates this type of prayer because it is most destructive of darkness. He then further counsels the pilgrim on practical methods to obtain pure prayer. The Pilgrim's first great spiritual breakthrough results in a “whole day experiencing great happiness and a complete detachment from earthly things.” After a summer of spiritual guidance from the monk, the Pilgrim purchases his own copy of the Philokalia and then wanders throughout Russia with the Jesus Prayer in his detached mind and heart.
The Jesus Prayer takes various forms, but its most common form is probably: "Lord Jesus Christ, son of God, have mercy on me, a sinner."
For more about the Jesus Prayer, see J.D. Salinger's Franny and Zooey, the book that first introduced me to it.
The Jesus Prayer is "so simple that it is within the reach of the humblest worshipper, yet so penetrating that it can introduce those who use it faithfully into the deepest mysteries of the contemplative life." Evelyn Underhill
The Jesus Prayer "saves one from many uncharitable, vain words or thoughts: it sanctifies one's daily toil and relationships." Nadejda Gorodetsky