This is the dense, brilliant center of the passage, and the wordplay is easy to miss in English. The word translated healthy is haplous, which does not simply mean physically well; its most natural sense is single, undivided, and generous. The word for bad is poneros, which can mean sick but also carries the weight of evil, jealous, and stingy. Jesus has chosen psychological language to make a moral point. The eye that is single minded and generous is full of light. The eye that is covetous, grasping, and stingy is full of darkness.
Throughout Scripture, the heart and the eyes function almost interchangeably as the direction of the inner person. The psalmist prays, “With my whole heart I seek you,” then, “Open my eyes, that I may behold wondrous things,” then, “Incline my heart to your testimonies,” then, “Turn my eyes from looking at worthless things” (Psalm 119:10, 18, 36, 37). Where our eyes settle reveals where our hearts have already gone. The eye that can only see earthly wealth is dark, no matter how prosperous the life looks from the outside.
Then comes the haunting line: if the light in you is darkness, how great is that darkness. The most dangerous darkness is the kind you do not know you are in. If the very standard by which you see is corrupted, you cannot diagnose your own blindness. This is precisely why Jesus warns about greed roughly ten times more than sexual sin; you almost always know when you are committing adultery, but you almost never know the moment money has quietly become your god.