Jesus uses the eye as a metaphor for the heart's focus or motive. Commentators explain that a 'single' eye is one that is healthy, clear, and fixed on one object: God and His heavenly kingdom. Just as a tightrope walker stays steady by fixing their gaze, a Christian's life is filled with 'light'—clarity, stability, and right conduct—when their spiritual focus is undivided. A divided focus on both God and worldly things leads to a life of 'darkness' and confusion.