“single.” The Greek word translated “single” is haplous (#573 ἁπλοῦς), and means “single,” therefore “unmixed.” The key to this saying about the “single” eye and the “evil eye,” in this context of wealth, is to realize they are Semitic idioms. In this context the “single eye” is the generous eye, it is unmixed with worldly desires for wealth and possessions, and is therefore generous towards others. In contrast, the “evil eye,” is used idiomatically in the Semitic languages for a person who is greedy, covetous, and envious. Not content with what he has, he casts his eye upon the things that others have, and desires them. The well-known Semitic scholar John Lightfoot writes that the saying about the single and evil eye is “From a very usual manner of speech of the [Jewish] nation. For a good eye, to the Jews, is the same with a bountiful mind; and an evil eye is the same with a covetous mind. (Commentary on the New Testament from the Talmud and Hebraica; Vol. 2, p. 156. Emphasis his).
Romans 12:8 says that the person who gives must give with “singleness” (haplotēs), again, idiomatically meaning “generously.” In James 1:5, the related word haplōs (simplicity, openness) is used idiomatically for “generously.”
It is easy to see how the words “good eye,” or “single eye,” became connected with generosity, and was used idiomatically for someone who was generous. The single eye was an eye that was unmixed with ulterior or selfish motives, and so the person was generous.
In Western cultures, the “evil eye” was a look or glance that meant harm and brought harm. Although this use of the “evil eye” may have existed in ancient Judaism, there is no reason to think it is used in Matthew or Luke. The Semitic idiom of the “good” or “single” eye being generous, and the “evil eye” being greedy, covetous, and stingy, holds true throughout the Bible. The “good eye” of Proverbs 22:9 is generous, and the “evil eye” of Deuteronomy 15:9; 28:54; Proverbs 23:6; 28:22 refers to someone who is greedy and stingy.
Once we understand that the “single eye” is generous, and the “evil eye” is greedy and envious, we can see why Jesus used it in this context. Jesus starts in Matthew 6:19 talking about laying up treasures in heaven, not on earth, and to do that one must have a single eye and be generous, and not have an evil eye and be greedy. Then he explains that no one can have two masters: you cannot try to serve heaven and earth. You cannot effectively love both God and wealth. The dialogue develops from there: if you are truly trying to serve God, then you cannot be worried about your earthly possessions. You must let go of your love for them and trust God to meet your needs.