From Plato's Republic

And now I will describe in a figure the enlightenment or un-enlightenment of our nature:

--imagine human beings living in an underground den which is open towards the light; they have been there from childhood, having their necks and legs chained, and can only see into the den. At a distance there is a fire, and between the fire and the prisoners a raised way, and a low wall is built along the way, like the screen over which marionette players show their puppets.

Behind the wall appear moving figures, who hold in their hands various works of art, and among them images of men and animals, wood and stone, and some of the passers-by are talking and others silent.

'A strange parable,' he said, 'and strange captives.'

They are ourselves, I replied; and they see only the shadows of the images which the fire throws on the wall of the den; to these they give names, and if we add an echo which returns from the wall, the voices of the passengers will seem to proceed from the shadows.

Suppose now that you suddenly turn them round and make them look with pain and grief to themselves at the real images; will they believe them to be real?

Will not their eyes be dazzled, and will they not try to get away from the light to something which they are able to behold without blinking?

And suppose further, that they are dragged up a steep and rugged ascent into the presence of the sun himself, will not their sight be darkened with the excess of light?

Some time will pass before they get the habit of perceiving at all; and at first they will be able to perceive only shadows and reflections in the water; then they will recognize the moon and the stars, and will at length behold the sun in his own proper place as he is.

Last of all they will conclude:--This is he who gives us the year and the seasons, and is the author of all that we see. How will they rejoice in passing from darkness to light!

How worthless to them will seem the honors and glories of the den!

But now imagine further, that they descend into their old habitations;--in that underground dwelling they will not see as well as their fellows, and will not be able to compete with them in the measurement of the shadows on the wall; there will be many jokes about the man who went on a visit to the sun and lost his eyes, and if they find anybody trying to set free and enlighten one of their number, they will put him to death, if they can catch him.