Theophylact of Ohrid — as excerpted in the Catena Aurea on Luke 11:33-36

“But as if the eye of the body be light the body will be light, but if dark the body will be dark also, so is it with the understanding in relation to the soul. Hence it follows, If thine eye be single, thy whole body will be full of light; but if evil, thy whole body will be full of darkness.”