SINGLE EYE (Gr. ophthalmos haplous, a healthy eye). Used in the KJV for an eye that is clear, sound, and healthy, with the connotation “generous” (Matt.6.22; Luke.11.34). NIV has “eyes are good.”
International Standard Bible Encyclopedia (1915)
Mt 6:22 f parallel Lu 11:34: "If therefore thine eye be single, thy whole body shall be full of light. But if thine eye be evil, thy whole body shall be full of darkness." "Single" and "evil" here represent haplouis, and poneros. Poneros elsewhere in the New Testament means "wicked"; haplous occurs only here in the New Testament, but is very common in ordinary Greek and always has the meaning "simple." But in view of the context, most commentators take haplous here as meaning "normal," "healthy," and poneros as "diseased," so rendering "Just as physical enlightenment depends on the condition of the eye, so does spiritual enlightenment depend on the condition of the heart." This is natural enough, but it is not satisfactory, as it gives to haplous a unique sense and to poneros a sense unique in the 73 New Testament examples of the word. Moreover, the same expression, "evil eye," is found also in Mt 20:15; Mr 7:22, where it means "jealousy" or "covetousness." With poneros = "covetous" haplous would = "generous"; and this rendition gives excellent sense in Matthew, where the further context deals with love of money. Yet in Luke it is meaningless, where the context is of a different sort, a fact perhaps indicating that Luke has placed the saying in a bad context. Or the Greek translation of Christ’s words used by Matthew and Luke may have taken the moral terms haplous and poneros to translate physical terms ("healthy" and "diseased"?) employed in the original Aramaic. The Sinaitic Syriac version of Lu 11:36 may perhaps contain a trace of an older rendering. See Julicher, Die Gleichnisreden Jesu, II, 98-108.