Gregory the Great
Catena Aurea · Lukasevangelium 11
No man, when he hath lighted a candle, putteth it in a secret place, neither under a bushel, but on a candlestick, that they which come in may see the light. The light of the body is the eye: therefore when thine eye is single, thy whole body also is full of light; but when thine eye is evil, thy body also is full of darkness. Take heed therefore that the light which is in thee be not darkness. If thy whole body therefore be full of light, having no part dark, the whole shall be full of light, as when the bright shining of a candle doth give thee light.
(28. Mor. c. 12.) Or else; By the name body each particular action is understood which follows its own intention, as it were the eye of the spectators. Therefore it is said, The light of the body is the eye, because by the ray of a good intention the deserving parts of an action receive light. If then thy eye be single, thy whole body will be full of light, for if we intend rightly in singleness of heart, we accomplish a good work, even though it seem not to be good. And if thy eye be evil, thy whole body will be full of darkness, because when with a crooked intention even a right thing is done, although it appears to glitterin men’s sight, yet before the bar of the internal judge it is covered with darkness. Hence too it is rightly added, Take heed therefore that the light which is in thee be not darkness. For if what we think we do well we cloud by a bad intention, how many are the evils themselves which even when we do them we know to be evil?