The Savior of the world has said, “The lamp of the body is the eye.” By lamp the Savior meant the spiritual power of the human soul, the spirit of man; by body the Savior meant all man’s activity and the quality of his life that is formed by and depends on this activity….
“When your eye is good,” that is, when the spiritual power is unconfused by sin or fellowship with Satan, then “your whole body also is full of light,” that is, your activity will be right and will be holy in quality. “But when your eye is bad, your body also is full of darkness.” See to it that your spirit, which is your natural light and the source of light for your life, does not become dark and a source of darkness.
This eye becomes evil through accepting falsehood. A result of this acceptance is wrong activity, while the quality of life becomes a state of sinfulness and self-deception. By accepting false thoughts the mind is corrupted, the conscience loses its reliability, and all the spiritual feelings of the heart are likewise infected with abnormality and sinfulness. Man becomes useless, an enemy of his own salvation, a murderer of his own soul, an enemy of God.
~Ignatius Brianchaninov
Prayerful Life 