Matthew 6:23 - Ellicott's Commentary On The Whole Bible

If thine eye be evil. — If the spiritual faculty, whose proper work it is to give light, be itself diseased — if it discerns not singly but doubly, and therefore dimly — then the whole life also is shrouded in gloom. If that is the case with the higher life, what will be the state of the lower! If the light is darkened, what will be the state of the region of life which is in itself naturally dark — the region of appetites and passions, which needs the presence of the light to keep them at all in check! “If the light that is in thee be darkness, the darkness how great will it be!”