“The eye is the lamp of the body. So, if your eye is sound, your whole body will be full of light; but if your eye is not sound, your whole body will be full of darkness. If then the light in you is darkness, how great is the darkness. No one can serve two masters; for either he will hate the one and love the other or he will be devoted to the one and despise the other. You cannot serve God and mammon. Matthew 6:22-24
“Jesus gives them the parable of the single eye. From single we move to sincere, sound and finally generous. The man who fixes his eyes on God (heaven), will give generously without second thought. The man who tries to look at God and the world at the same time will see neither clearly; in fact he will not see at all.”
F.F. Bruce in The International Standard Bible Commentary (Grand Rapids, MI: Zondervan, 1979) 1128.