Imagine prisoners who have been chained since their childhood deep inside a cave: not only are their arms and legs unmovable because of chains; their heads are chained in one direction 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 puppets of various animals, plants, and other things are moved along. The puppets cast shadows on the wall, and the prisoners watch these shadows. Behind this cave there is a well-used road, and upon this road people are walking and talking and generally making noise. The prisoners, then, believe that these noises are coming directly from the shadows they are watching pass by on the cave wall.
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 objects. They are thus conditioned to judge the quality of one another by their skill in quickly naming the shapes and dislike those who play poorly.
Suppose a prisoner's chains break, and he is able to get up and walk about (a process which takes some time, as he has never done it before). Eventually he will be compelled to explore; he walks up and out of the cave, whereby he is instantly blinded by the sun. He turns then to the shadows on the floor, in the lakes, slowly working his way out of his deluded mind, and he is eventually able to glimpse the sun. In time, he would learn to see it as the object that 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.
Once enlightened, so to speak, the freed prisoner would not want to return to the cave to free his fellow prisoners, but would be compelled to do so. Another problem lies in the other prisoners not wanting to be freed: descending back into the cave would require that the freed prisoner's eyes adjust again, and for a time, he would be one of the ones identifying shapes on the wall. His eyes would be swamped by the darkness, and would take time to become acclimated. He might stumble, and the prisoners would conclude that his experience had ruined him. He would not be able to identify the shapes on the wall as well as the other prisoners, making it seem as if his being taken to the surface completely ruined his eyesight.
