The lamp of the body is the eye.
“The lamp of the body is the eye; if your eyes are sound, your whole body will be full of light”.
Such detachment makes possible the generosity to which Jesus invites us in the saying that follows (6:22) about the ‘good’ and the ‘bad’ (or ‘evil’) eye.
In Jewish tradition having a good eye means being a generous person.
Having an evil eye, means being mean and niggardly.
The saying presupposes the ancient theory of light: that the eye sees, not from receiving reflected light from outside, (but like the beam of a torch) by illuminating objects with light from within itself.
If our heart is sound, that is, compassionate, then we shall radiate light (benevolence) around us.
If our heart is tied up to its treasures on earth (and so, closed to compassion), then we shall treat people around us in a mean and miserly way – we will be ‘evil’.