THE MAN BORN BLIND
THERE was a man born blind, and he said: "I do not believe in the world of light and appearance. There are no colors, bright or somber. There is no sun, no moon, and no stars. No one has witnessed these things." His friends remonstrated with him, but he clung to his opinion: "What you say that you see," he objected, "are illusions. If colors existed I should be able to touch them. They have no substance and are not real. Everything real has weight, but I feel no weight where you see colors."
A physician was called to see the blind man. He mixed four simples, and when he applied them to the cataract of the blind man the gray film melted, and his eyes acquired the faculty of sight. The Tathagata is the physician, the cataract is the illusion of the thought "I am," and the four simples are the four noble truths.
Taken from: http://www.sacred-texts.com/bud/btg/btg65.htm