The light of the body is the eye: if therefore your eye be sound, your whole body shall be full of light.
Read Chapter 6
Augustine of Hippo
AD 430
And here He makes it manifest that He gives all these precepts with a view to the cleansing of the heart.
And this passage we are to understand in such a way as to learn from it that all our works are pure and well-pleasing in the sight of God, when they are done with a single heart, i.e. with a heavenly intent, having that end of love in view; for love is also the fulfilling of the law. Hence we ought to take the eye here in the sense of the intent itself, wherewith we do whatever we are doing; and if this be pure and right, and looking at that which ought to be looked at, all our works which we perform in accordance therewith are necessarily good. And all those works He has called the whole body; for the apostle also speaks of certain works of which he disapproves as our members, and teaches that they are to be mortified, saying, Mortify therefore your members which are upon the earth; fornication, uncleanness, covetousness, and all other such things.
Every action is lighted or directed by the intention. If the intention be upright, the whole body of the action is good, provided it proceed not from a false conscience. If the intention be bad, how bad must be the action! Christ does not here speak of an exterior, but an interior eye. He, therefore, who directs all his thoughts to God, may justly be said to have his eye lightsome, and consequently his heart undefiled with worldly affections; but he who has all his thoughts corrupted with carnal desires is, beyond a doubt, enveloped in darkness. (St. Chrysostom)
The single eye is the love unfeigned; for when the body is enlightened by it, it sets forth through the medium of the outer members only things which are perfectly correspondent with the inner thoughts. But the evil eye is the pretended love, which is also called hypocrisy, by which the whole body of the man is made darkness. We have to consider that deeds meet only for darkness may be within the man, while through the outer members he may produce words that seem to be of the light: for there are those who are in reality wolves, though they may be covered with sheep's clothing. Such are they who wash only the outside of the cup and platter, and do not understand that, unless the inside of these things is cleansed, the outside itself cannot be made pure. Wherefore, in manifest confutation of such persons, the Saviour says: If the light that is in you be darkness, how great is that darkness! That is to say, if the love which seems to you to be light is really a work meet for darkness, by...
However, forasmuch as this was too high for the mind of His hearers, and neither was the mischief within easy view of the generality, nor the gain evident, but there was need of a spirit of more self-command to perceive either of these; first, He has put it after those other topics, which are obvious, saying, Where the man's treasure is, there is his heart also; and next He makes it clear again, by withdrawing His discourse from the intellectual to the sensible, and saying,
The light of the body is the eye.
What He says is like this: Bury not gold in the earth, nor do any other such thing, for thou dost but gather it for the moth, and the rust, and the thieves. And even if you should entirely escape these evils, yet the enslaving of your heart, the nailing it to all that is below, you will not escape: For wheresoever your treasure may be, there is your heart also. As then, laying up stores in heaven, you will reap not this fruit only, the attainment of the rewards for these thing...
This means, if you fill your mind with worries over money, you have extinguished the lamp and darkened your soul. Just as the eye that is sound, or "healthy" brings light to the body, and the eye that is evil, or "diseased" brings darkness, so also does the state of the mind affect the soul. If the mind is blinded by these worries, it is cast into darkness; then the soul becomes dark, and how much more so the body as well?
