Plato, The Republic (Book VII):
Socrates: Now compare our condition with this: Picture men living in a cave which has a wide mouth open towards the light. They are kept in the same places, looking forward only away from the mouth and unable to turn their heads, for their legs and necks have been fixed in chains from birth. A fire is burning higher up at their backs, and between it and the prisoners there is a road with a low wall built at its side, like the screen over which puppet players put up their puppets.
Glaucon: All that I see.
Socrates: See, again, then, men walking under cover of this low wall carrying past all sorts of things, copies of men and animals, in stone or wood and other materials; some of them may be talking and others not.
Glaucon: This is a strange sort of comparison and these are strange prisoners.
Socrates: They are like ourselves. They see nothing but their own shadows, or one another's, which the fire throws on the wall of the cave. And so too with the things carried past. If they were able to talk to one another, wouldn't they think that in naming the shadows they were naming the things that went by? And if their prison sent back an echo whenever one of those who went by said a word, what could they do but take it for the voice of the shadow?
Glaucon: By Zeus, they would.
Socrates: The only real things for them would be the shadows of the puppets.
Glaucon: Certainly.
Socrates: Now see how it will be if something frees them from their chains: When one is freed and forced to get on his feet and turn his head and walk and look towards the light -- and all this hurts, and because the light is too bright, he isn't able to see the things whose shadows he saw before -- what will he answer, if someone says that all he has seen till now, was false and a trick, but that now he sees more truly? And if someone points out to him the things going by and asks him to name them, won't he be at a loss? And won't he take the shadows he saw before as more real than these things?
Glaucon: Much more real.
Socrates: And if he were forced to look straight at the light itself, wouldn't he start back with pained eyes? And if someone pulled him up the rough and hard ascent and forced him out into the light of the sun, wouldn't he be angry? And wouldn't his eyes be too full of light to make out even one of the things we say are real?
Glaucon: Yes, that would be so at first.
Socrates: He would need to get used to the light before he could see things up there. At first he would see shadows best, and after that reflections in still water of men and other things, and only later these things themselves. Then he would be ready to look at the moon and stars, and would see the sky by night better than the sun and the sun's light by day. So, at last, I take it, he'd be able to look upon the sun itself, and see it not through the seemings and images of itself in water and away from its true place, but in its own field and as it truly is.
Glaucon: So. [meaning, "it is so," not "so what?"]
Socrates: And with that he will discover that it is the sun which gives the seasons and the years, and is the chief in the field of the things which are seen, and in some way the cause even of all the things he had been seeing before. If he now went back in his mind to where he was living before, and to what his brother slaves took to be wisdom there, wouldn't he be happy at the change and pity them?
Glaucon: Certainly, he would.
Socrates: And if their way was to reward those who were quickest to make out the shadows as they went by and to note in memory which came before as a rule, and which together, would he care very much about such rewards? And, if he were to go down again out of the sunlight into his old place, would not his eyes get suddenly full of the dark? And if there were to be a competition then with the prisoners who had never moved out and he had to do his best in judging the shadows before his eyes got used to the dark -- which needs more than a minute -- wouldn't he be laughed at? Wouldn't they say he had come back from his time on high with his eyes in very bad condition so that there was no point in going up there? And if they were able to get their hands on the man who attempted to take their chains off and guide them up, wouldn't they put him to death?
Glaucon: They certainly would!
Socrates: Take this comparison, dear Glaucon, with all we have said before. The world seen through the eyes, that is the prison house; the light of the fire is like the power of the sun; and if you see the way out and that looking upon things of the upper world as the going up of the soul to the field of true thought, you will have my hopes or beliefs about it and they are what you desired -- though only God knows if they are right. Be that as it may, what seems clear to me is that in the field of deep knowledge the last thing to be seen, and hardly seen, is the idea of the good. When that is seen, our decision has to be that it is truly the cause, for all things, of all that is beautiful and right. In the world that is to be seen, it gives birth to light and to the lord of light, but in the field of thought it is itself the master cause of reason and all that is true; and anyone who is to act wisely in private or public must have seen this...