Whenever I don't understand what a passage is talking about--which happens frequently with the sayings of Jesus--I try to start with what I do understand and take it step by step from there. This is one of those.
This passage comes after Jesus' teaching that you should lay up treasures in heaven and not on earth, and comes before his teaching that you cannot serve two masters. There's a common theme here. You cannot live for both heaven and earth, for both God and money. You have to choose. Where does your heart's treasure really lie? It's obvious that today's passage, which is sandwiched right in between, must also be addressing this theme.
That means Jesus' analogy about the eye's relationship to the body has to be illustrating this theme. Your eye is the lamp of the body. If your eye is clear your whole body is full of light. But if your eye is bad then your whole body is full of darkness. In other words if your eye can see clearly your whole body knows where it's going, but if your eye is blind you essentially exist in darkness. When you think about it, there isn't a practical difference between being in a dark room versus being in a lit room with your eyes shut. Once your eye shuts out the light, you are in darkness. Your body doesn't know where to go or what to do. But when your eye takes in the light, your body is also "sees" clearly.
How does the eye/light/body analogy fit with the theme of making the right choice about where your true treasure lies? I see two possibilities that center around the Greek word haplous which my Bible translates as "clear." What does it mean to have a haplous or clear eye?
I am told that a literal translation of haplous is "single" or "undivided," as in undivided loyalty. This seems to fit into Jesus' overall exhortation quite nicely because then he's saying that having a "single eye" is having an undivided loyalty to God, as opposed to trying to serve two masters or lay up treasure in two places. That single-eyed devotion brings light and clarity of purpose to the whole body, whereas the double-minded person dwells in darkness and doesn't know where their true treasure ought to lie.
I know that explanation sounds really convincing but there's also another equally compelling possibility. The word haplous can have the connotation--or the implied meaning--of "generous." For instance, the adverbial form of haplous shows up in James 1:5: "God who gives to all men generously (haplos)." So now we have to consider that Jesus may be talking about having a "generous eye." What's more the term "bad" that Jesus uses when he says "if your eye is bad" could mean having an "evil eye," which is a common expression for jealousy, miserliness and ungenerosity. In short, Jesus could be saying that a generous eye gives light, clarity and purpose to the whole body, whereas an ungenerous eye causes the whole person to dwell in darkness.
And yet a third possibility--if you can believe it--may be that Jesus wants to imply both meanings and is using haplous as a play on words. Seeing your true master with a single-eyed loyalty enables you to be generous, whereas having divided loyalties will blind you with miserliness. One enlightens you with purpose and direction while the other leaves you wandering in confusion.
I'll leave you to make the call. But whatever nuance you find most convincing, getting the overall message of setting your sights on your true treasure and your true master is what's important.