The body finds its way through life with the aid of the eye. In that sense the eye is the lamp of the body (cf. Luke 11:34-36). A clear or good eye admits light into the body, but a bad eye leaves the body in darkness. Evidently Jesus meant the eye is similar to the heart ( Matthew 6:21). The heart fixed on God ( Psalm 108:1-2) is similar to the eye fixed on God"s law ( Psalm 119:18; Psalm 119:148). [source][source][source]
"Eyes are the expression of the soul, not its intake, although certainly the two ideas are related. What Jesus stresses in this saying is that a good eye acts in a healthy way. It is the sign of a healthy soul." [1][source]
A bad eye is a miserly eye ( Proverbs 28:22). Jesus was speaking metaphorically. He probably meant that the person who is stingy and selfish cannot really see where he is going but is morally and spiritually blind (cf. Matthew 6:19-21). [2] However, He may have meant that the person who is double-minded, dividing his loyalties between God and money, will have no clear vision but will lack direction (cf. Matthew 6:24). [3] Metaphorically the body represents the whole person. The light within is the vision that the eye with divided loyalties, a selfish attitude, provides. [source][source][source]