The lamp of the body is the eye. Therefore, if your eye should be single, your whole body will be full of light. But if your eye should be evil, your whole body will be full of darkness. Therefore, if the light in you is darkness, how great is the darkness!
--Matthew 6:22-23, literal translation
The above teaching is given by Jesus in the Sermon on the Mount. Unfortunately, its true meaning passes the average reader by, mainly because of the Jewish idioms "single eye" and "evil eye." Many modern translations seem to miss the point entirely when they take these phrases to have a medical meaning: "eye is sound," "eye is not sound" (RSV); "eye is clear." "Eye is bad" (NASV); "eyes are good," "eyes are bad" (NIV). Surely Jesus is not saying that those with eye trouble are full of great darkness!
The key to understanding the phrase "single eye" is to see how the noun "singleness" is used in connection with passages on giving. In Romans 12:8 it is translated "simplicity" (KJV) or "liberality" (RSV). In II Corinthians 9:11 it is translated "bountifulness" (KJV) or "generosity" (RSV), and in verse 13 "liberal" (KJV) or "generosity" (RSV). It is not hard to see that in a context of giving the word "single" can mean "generous." Thus if the passage is talking about giving and/or sharing with others, one could translate, "if your eye is generous," i.e., if you are generous, you are full of spiritual light.
But is the context speaking of giving and sharing? Yes, it is. This is shown by looking at other passages containing the expression "evil eye." For example, it is found in Deuteronomy 15:7-10:
"If there is among you a poor man, one of your brethren, in any of your towns within your land which the LORD your God gives you, you shall not harden your heart or shut your hand against your poor brother, but you shall open your hand to him, and lend him sufficient for his need, whatever it may be. Take heed lest there be a base thought in your heart, and you say, 'The seventh year the year of release is near,' and your eye be hostile [Literally, evil, emphasis mine--rbt] to your poor brother, and you give him nothing, and he cry to the LORD against you, and it be sin in you. You shall give to him freely, and your heart shall not be grudging when you give to him; because for this the LORD your God will bless you in ali your work and in all that you undertake." (RSV)
Here it is obvious that an evil eye is a grudging eye and the context is sharing. Or again, in the Apocrypha, the writings between the testaments, in Sirach 14:8, we read, "The grudging man has an evil eye." Or still again, in the parable of the householder in Matthew 20:1-16, when the householder paid the last workers the same wages as the first, he said to the complaining workers, "Is your eye evil because I am good," i.e., "are you grudging because I am good?" In addition, Mark 7:21-22 lists the evil eye as coming from within the heart of man. Thus the phrase "if your eye is evil" could be translated "if your eye is grudging," i.e., if you are grudging, you are full of spiritual darkness.
Thus Jesus is speaking of the generous man and the grudging man. Which are you? Is your eye single (generous) or evil (grudging)? --rbt