But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great [is] that darkness!
Note 40 at Mt 6:23: Jesus was contrasting an evil eye with the single eye just mentioned (Mt 6:22). An evil eye is simply when we allow our thoughts to be anything less than single upon Jesus (Jas 1:8).
Note 41 at Mt 6:23: As light is symbolic of God and His kingdom (Isa 9:2; Joh 1:4-9, 8:12; and 1Jo 1:5), so darkness symbolizes Satan and his kingdom (Joh 1:5, 3:19; Ro 13:12; 2Co 6:14; 1Jo 1:6, and 2:11). Not having our minds stayed on Jesus gives Satan an inroad into our lives (Ro 8:6). If we are setting our affections on things of this earth (Col 3:1-4), then Satan and the unfruitful works of darkness (Eph 5:11) will dominate us.