Gregory the Great — as excerpted in the Catena Aurea on Luke 11:33-36

“(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 glitter in 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?”