Just a short note on this very difficult saying. Although it is between two sayings on wealth, the connection to wealth is not clear to most modern readers. First, ancient people thought the eye illuminated the mind. The science is not right in the analogy from a modern perspective, but it is from the first century way of thinking about how an eye works. Second, “healthy” and “unhealthy” are metaphors for generosity and stinginess in Deuteronomy 15:9 (“your eye is wicked” (וְרָעָ֣ה עֵֽינְךָ֗), LXX: καὶ πονηρεύσηται ὁ ὀφθαλμός σου τῷ ἀδελφῷ σου) and Proverbs 22:9 “a good eye” (טֽוֹב־עַ֭יִן).
Deuteronomy 15:9 (ESV) Take care lest there be an unworthy thought in your heart and you say, ‘The seventh year, the year of release is near,’ and your eye look grudgingly on your poor brother, and you give him nothing, and he cry to the Lord against you, and you be guilty of sin.
Proverbs 22:9 (ESV) Whoever has a bountiful eye will be blessed, for he shares his bread with the poor.
Third, In Matthew 20 Jesus tells a parable of workers in the field. Those who grumble about the owner’s generosity are envious (NIV) or “begrudge” the generosity of the owner (ESV). Literally, this is “your eye is evil because I am good” (ὁ ὀφθαλμός σου πονηρός ἐστιν).
A “good eye” and a “bad eye” are therefore contrasting metaphors for how one perceives wealth. A generous person who does not hold their possessions too closely is a light in the world, in contrast to the utter darkness of the one who hoards wealth.