Imagine prisoners who have been chained since childhood deep inside a cave. Not only are their limbs immobilized by the chains; their heads are chained as well so that their gaze is fixed on a wall.
Behind the prisoners is an enormous fire, and between the fire and the prisoners is a raised walkway, along which shapes of various animals, plants, and other things are carried. The shapes cast shadows on the wall, and the prisoners watch these shadows. When one of the shape-carriers speaks, an echo against the wall causes the prisoners to believe that the words come from the shadows.
The prisoners engage in what appears to us to be a game - naming the shapes as they come by. This, however, is the only reality that they know, even though they are seeing merely shadows of images. They are thus conditioned to judge the quality of one another by their skill in quickly naming the shapes and dislike those who begin to play poorly.
Suppose a prisoner is released and compelled to stand up and turn around.
His eyes will be blinded by the firelight, and the shapes passing will appear less real than their shadows.
Similarly, if he is dragged up out of the cave into the sunlight, his eyes will be so blinded that he will not be able to see anything. At first, he will be able to see darker shapes such as shadows and, only later, brighter and brighter objects.
The last object he would be able to see is the sun, which, in time, he would learn to see as that object which provides the seasons and the courses of the year, presides over all things in the visible region, and is in some way the cause of all these things that he has seen.
