Matthew 6:22 – 23

22 “The lamp of the body is the eye. If therefore your eye is good, your whole body will be full of light.

23 But if your eye is bad, your whole body will be full of darkness. If therefore the light that is in you is darkness, how great is that darkness!

Jesus uses the body as an example, the eye being the part used to relay his message to us. The eye is the only part of the body which allows light into the body, and its use of light is needed for it to operate successfully, with out light the body is blind to its surroundings. A good eye allows all the light in its environment to be used to optically evaluate the surroundings of the body.

If the eye is bad then it is blind to its surroundings, which means light is useless to it.

In this example He is explaining how the soul needs the spiritual light he provides for the body(soul) to function righteously. The man's intellect or mind is the only means of receiving spiritual light which is what Jesus brings into the world. (John 1:4 – 5)(John 3:19 – 21) Man is living in darkness if he refuses to live in the light which Jesus brings to the world. If man refuses to use the light given him, how great is the darkness he is living in, he is spiritually blind and unable to follow the path given him to obtain eternal life.