“The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light.”
Matthew 6:22 — Greek Interlinear
Church Fathers on Matthew 6:22
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…
That is an illustration drawn from the senses. As the whole body is in darkness, where the eye is not single, so if the soul has lost her original brightness, every sense, or that whole part of the soul to which sensation belongs, will abide in darkness.
It seems that He is not here speaking of the bodily eye, or of the outward body that is seen, or He would have said, If thine eye be sound, or weak; but He says, 'single,' and, 'evil.' But if one have a benign yet diseased eye, is his body therefore in light? Or if an evil yet a sound, is his body therefore in darkness?