Matthew 6:22-23 and Luke 11:34-36 are confusing for moderns. We think of the eye as a window, letting in light. But ancients could think of an eye another way, as lights shining the interior light of the body. There are many examples in the Bible and in Greek texts. Psalm 38:11 is one example, “My eyes too have lost their luster” (see also Prov 15:30).
This has nothing to do with the scientific or pre-scientific way of looking at vision. It is the observation, which moderns also make, that a person’s eyes show their inner character. So what does Yeshua’s saying mean and what does it have to do with the evil eye?
Yeshua contrasts two kinds of eyes. The problem is knowing how to translate best the first word. It can mean single, healthy, or possibly generous. It is contrasted with the evil eye. The concept of the evil eye is well known in rabbinic and pagan texts as an idiom for one giving a curse. So, Yeshua’s meaning is to contrast the eye that curses others with the eye that blesses or desires good for others. Generous is a good translation (possibly healthy could work as well, though the understanding is that the healthy eye sees people as needing blessing).
So, Yeshua’s saying now makes sense. If the eye is generous, it is bright and the body is filled with interior light or goodness. But if the eye is set on cursing others, the body is full of darkness. In simplest terms, Yeshua is saying that generosity is the ultimate sign of a righteous disciple.