Matteo 6:23

But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness!

The absurdity and dangerousness of covetousness is here illustrated, probably with reference to the Pharisees, whose attention and affections were divided between temporal and spiritual things, and who therefore became spiritually blind. The eye is the organ of vision and incidentally the seat of expression. To perform its function properly, it should be the light of the body, give light for the body's movement and labor.

The candid, open, healthy eye will give this service properly; the bad, diseased eye will cause the whole body to be in darkness, though the person stand in the midst of light. In other words: The light of the body is the eye, because the eye lets light into the body and makes it available to the body. When the eye of the soul is in proper condition, free from the desire to hoard, then true Christian knowledge can control and direct the person unto every good work.

But when sordid passions take hold of the soul, Christian knowledge suffers, heart and mind are blinded, judgment is perverted, and nothing but evil results. There is spiritual darkness without a single ray of light, just as the extinguishing of a lamp in a dark room intensifies the darkness greatly.