[translation from http://etext.library.adelaide.edu.au/p/plato/p71r/introduction7.html, with paragraph breaks added by Annalisa Crannell]

And now I will describe in a figure the enlightenment or unenlightenment 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 honours 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.

Now the cave or den is the world of sight, the fire is the sun, the way upwards is the way to knowledge, and in the world of knowledge the idea of good is last seen and with difficulty, but when seen is inferred to be the author of good and right—parent of the lord of light in this world, and of truth and understanding in the other. He who attains to the beatific vision is always going upwards; he is unwilling to descend into political assemblies and courts of law; for his eyes are apt to blink at the images or shadows of images which they behold in them—he cannot enter into the ideas of those who have never in their lives understood the relation of the shadow to the substance. But blindness is of two kinds, and may be caused either by passing out of darkness into light or out of light into darkness, and a man of sense will distinguish between them, and will not laugh equally at both of them, but the blindness which arises from fulness of light he will deem blessed, and pity the other; or if he laugh at the puzzled soul looking at the sun, he will have more reason to laugh than the inhabitants of the den at those who descend from above.

There is a further lesson taught by this parable of ours. Some persons fancy that instruction is like giving eyes to the blind, but we say that the faculty of sight was always there, and that the soul only requires to be turned round towards the light. And this is conversion; other virtues are almost like bodily habits, and may be acquired in the same manner, but intelligence has a diviner life, and is indestructible, turning either to good or evil according to the direction given. Did you never observe how the mind of a clever rogue peers out of his eyes, and the more clearly he sees, the more evil he does? Now if you take such an one, and cut away from him those leaden weights of pleasure and desire which bind his soul to earth, his intelligence will be turned round, and he will behold the truth as clearly as he now discerns his meaner ends. And have we not decided that our rulers must neither be so uneducated as to have no fixed rule of life, nor so over-educated as to be unwilling to leave their paradise for the business of the world?

We must choose out therefore the natures who are most likely to ascend to the light and knowledge of the good; but we must not allow them to remain in the region of light; they must be forced down again among the captives in the den to partake of their labours and honours. ‘Will they not think this a hardship?’ You should remember that our purpose in framing the State was not that our citizens should do what they like, but that they should serve the State for the common good of all. May we not fairly say to our philosopher,—Friend, we do you no wrong; for in other States philosophy grows wild, and a wild plant owes nothing to the gardener, but you have been trained by us to be the rulers and kings of our hive, and therefore we must insist on your descending into the den. You must, each of you, take your turn, and become able to use your eyes in the dark, and with a little practice you will see far better than those who quarrel about the shadows, whose knowledge is a dream only, whilst yours is a waking reality. It may be that the saint or philosopher who is best fitted, may also be the least inclined to rule, but necessity is laid upon him, and he must no longer live in the heaven of ideas.

And this will be the salvation of the State. For those who rule must not be those who are desirous to rule; and, if you can offer to our citizens a better life than that of rulers generally is, there will be a chance that the rich, not only in this world’s goods, but in virtue and wisdom, may bear rule. And the only life which is better than the life of political ambition is that of philosophy, which is also the best preparation for the government of a State.