excerpted from Book VII of Plato's Republic:

“Next,” said I, “compare our nature in respect of education and its lack to such an experience as this. Picture men dwelling in a sort of subterranean cavern1 with a long entrance open2 to the light on its entire width. Conceive them as having their legs and necks fettered3 from childhood, so that they remain in the same spot, 514b able to look forward only, and prevented by the fetters from turning their heads. Picture further the light from a fire burning higher up and at a distance behind them, and between the fire and the prisoners and above them a road along which a low wall has been built, as the exhibitors of puppet-shows4 have partitions before the men themselves, above which they show the puppets.” “All that I see,” he said. “See also, then, men carrying5 past the wall 514c implements of all kinds that rise above the wall, and human images 515a and shapes of animals as well, wrought in stone and wood and every material, some of these bearers presumably speaking and others silent.” “A strange image you speak of,” he said, “and strange prisoners.” “Like to us,” I said; “for, to begin with, tell me do you think that these men would have seen anything of themselves or of one another except the shadows cast from the fire on the wall of the cave that fronted them?” “How could they,” he said, “if they were compelled 515b to hold their heads unmoved through life?” “And again, would not the same be true of the objects carried past them?” “Surely.” “If then they were able to talk to one another, do you not think that they would suppose that in naming the things that they saw1 they were naming the passing objects?” “Necessarily.” “And if their prison had an echo2 from the wall opposite them, when one of the passersby uttered a sound, do you think that they would suppose anything else than the passing shadow to be the speaker?” “By Zeus, I do not,” said he. “Then in every way 515c such prisoners would deem reality to be nothing else than the shadows of the artificial objects.” “Quite inevitably,” he said. “Consider, then, what would be the manner of the release3 and healing from these bonds and this folly if in the course of nature4 something of this sort should happen to them: When one was freed from his fetters and compelled to stand up suddenly and turn his head around and walk and to lift up his eyes to the light, and in doing all this felt pain and, because of the dazzle and glitter of the light, was unable to discern the objects whose shadows he formerly saw, 515d what do you suppose would be his answer if someone told him that what he had seen before was all a cheat and an illusion, but that now, being nearer to reality and turned toward more real things, he saw more truly? And if also one should point out to him each of the passing objects and constrain him by questions to say what it is, do you not think that he would be at a loss5 and that he would regard what he formerly saw as more real than the things now pointed out to him?” “Far more real,” he said.

“And if he were compelled to look at the light itself, 515e would not that pain his eyes, and would he not turn away and flee to those things which he is able to discern and regard them as in very deed more clear and exact than the objects pointed out?” “It is so,” he said. “And if,” said I, “someone should drag him thence by force up the ascent6 which is rough and steep, and not let him go before he had drawn him out into the light of the sun, do you not think that he would find it painful to be so haled along, and would chafe at it, and when 516a he came out into the light, that his eyes would be filled with its beams so that he would not be able to see1 even one of the things that we call real?” “Why, no, not immediately,” he said. “Then there would be need of habituation, I take it, to enable him to see the things higher up. And at first he would most easily discern the shadows and, after that, the likenesses or reflections in water2 of men and other things, and later, the things themselves, and from these he would go on to contemplate the appearances in the heavens and heaven itself, more easily by night, looking at the light 516b of the stars and the moon, than by day the sun and the sun's light.3” “Of course.” “And so, finally, I suppose, he would be able to look upon the sun itself and see its true nature, not by reflections in water or phantasms of it in an alien setting,4 but in and by itself in its own place.” “Necessarily,” he said. “And at this point he would infer and conclude that this it is that provides the seasons and the courses of the year and presides over all things in the visible region, 516c and is in some sort the cause5 of all these things that they had seen.” “Obviously,” he said, “that would be the next step.” “Well then, if he recalled to mind his first habitation and what passed for wisdom there, and his fellow-bondsmen, do you not think that he would count himself happy in the change and pity them6?” “He would indeed.” “And if there had been honors and commendations among them which they bestowed on one another and prizes for the man who is quickest to make out the shadows as they pass and best able to remember their customary precedences, 516d sequences and co-existences,7 and so most successful in guessing at what was to come, do you think he would be very keen about such rewards, and that he would envy and emulate those who were honored by these prisoners and lorded it among them, or that he would feel with Homer8 and “‘greatly prefer while living on earth to be serf of another, a landless man,’”Hom. Od. 11.489 and endure anything rather than opine with them 516e and live that life?” “Yes,” he said, “I think that he would choose to endure anything rather than such a life.” “And consider this also,” said I, “if such a one should go down again and take his old place would he not get his eyes full9 of darkness, thus suddenly coming out of the sunlight?” “He would indeed.” “Now if he should be required to contend with these perpetual prisoners 517a in 'evaluating' these shadows while his vision was still dim and before his eyes were accustomed to the dark—and this time required for habituation would not be very short—would he not provoke laughter,1 and would it not be said of him that he had returned from his journey aloft with his eyes ruined and that it was not worth while even to attempt the ascent? And if it were possible to lay hands on and to kill the man who tried to release them and lead them up, would they not kill him2?” “They certainly would,” he said.

A link to the end of this passage (in which Socrates further explains the allegory): Perseus link You can click on the blue arrows at the top of the passage to move forward or backward for more context.