John Chrysostom
Catena Aurea · Matthäusevangelium 6
The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light. 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!
Having spoken of the bringing the understanding into captivity because it was not easy to be understood of many, He transfers it to a sensible instance, saying, The light of thy body is thy eye. As though He had said, If you do not know what is meant by the loss of the understanding, learn a parable of the bodily members; for what the eye is to the body, that the understanding is to the soul. As by the loss of the eyes we lose much of the use of the other limbs, so when the understanding is corrupted, your life is filled with many evils.