Matthew 6:22 & 23
"'The eye is the lamp of the body. If your eyes are healthy, your whole body will be full of light. But if your eyes are unhealthy, your whole body will be full of darkness. If then the light within you is darkness, how great is that darkness!'" NIV translation
The eyes speak of knowledge and, here, Jesus is talking about the dangers of materialism. If your eyes cannot see the danger of this, then, your whole body will chase after the things of this world. Chasing after the things of this world only leads to darkness. If you see earthly things as they really are, which is temporary, then your heart as well as your entire body will be focused on kingdom things.