The light of the body ... - 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.
Single - Steady, directed to one object. Not confused, as persons’ eyes are when they see double.
Thy body shall be full of light - Your conduct will be regular and steady. All that is needful to direct the body is that the eye be fixed right. No other light is required. So all that is needful to direct the soul and the conduct is, that the eye of faith be fixed on heaven; that the affections be there.
If, therefore, the light that is in thee ... - The word “light,” here, signifies “the mind,” or principles of the soul. If this is dark, how great is that darkness! The meaning of this passage may be thus expressed: The light of the body, the guide and director, is the eye. All know how calamitous it is when that light is irregular or extinguished, as when the eye is diseased or lost. So the light that is in us is the soul. If that soul is debased by attending exclusively to earthly objects - if it is diseased, and not fixed on heaven how much darker and more dreadful will it be than any darkness of the eye! Avarice darkens the mind, obscures the view, and brings in a dreadful and gloomy night over all the faculties.