_"Behold! human beings living in a sort of underground den, which has a mouth open towards the light and reaching all across the den; they have been here from their childhood, and have their legs and necks chained so that they cannot move, and can only see before them; for the chains are arranged in such a manner as to prevent them from turning round their heads. At a distance above and behind them the light of a fire is blazing, and between the fire and the prisoners there is a raised way; and you will see, if you look, a low wall built along the way, like the screen which marionette players have before them, over which they show the puppets.
I see, he said.
And do you see, I said, men passing along the wall carrying vessels, which appear over the wall; also figures of men and animals, made of wood and stone and various materials; and some of the prisoners, as you would expect, are talking, and some of them are silent?
This is a strange image, he said, and they are strange prisoners.
Like ourselves, I replied; and they see only their own shadows, or the shadows of one another, which the fire throws on the opposite wall of the cave?
True, he said: how could they see anything but the shadows if they were never allowed to move their heads?
And of the objects which are being carried in like manner they would see only the shadows?
Yes, he said.
And if they were able to talk with one another, would they not suppose that they were naming what was actually before them?"_ --The Republic of Plato, Book Seven. (Jowett Translation.)