Your eye is the lamp of your body. When your eyes are good, your whole body also is full of light. But when they are bad, your body also is full of darkness. In this verse, Jesus teaches that the fundamental reason why the Jews became sign seekers is that they have an eyesight problem. It is caused by an imbalance between the production and draining of the fluid produced by cells behind the lens of the eye.
People can lose eyesight completely.
But since no pain is involved, and the loss of eye-sight is so gradual, it may go unnoticed for a long time until we become totally blind. When people don't repent of their sins regularly, they become numb and gradually lose their spiritual insight so badly that the light of Jesus cannot reach in their hearts and souls, thus they remain in darkness. Look at verse 35. In short, they needed spiritual eye surgery. They should have a humble, learning mind like the Queen of the South and repent of their sins like the people of Nineveh before the word of God.