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
“The sentiment stated in the preceding verses - the duty of fixing the affections on heavenly things - Jesus proceeds to illustrate by a reference to the “eye.” When the eye is directed steadily toward an object, and is in health, or is single, everything is clear and plain. If it vibrates, flies to different objects, is fixed on no one singly, or is diseased, nothing is seen clearly. Everything is dim and confused. The man, therefore, is unsteady. The eye regulates the motion of the body. To have an object distinctly in view is necessary in order to correct and regulate action. Rope-dancers, that they may steady themselves, fix the eye on some object on the wall, and look steadily at that. If they should look down on the rope or the people, they might become dizzy and fall. A man crossing a stream on a log, if he will look across at some object steadily, will be in little danger. If he looks down on the dashing and rolling waters, he will become dizzy, and fall. So Jesus says, in order that the conduct may be right, it is important to fix the affections on heaven. Having the affections there - having the eye of faith single, steady, unwavering - all the conduct will be correspondent.” - Albert Barns