Plato's allegory of the cave is perhaps the best-known of his many metaphors, allegories, and myths. The allegory is told and interpreted at the beginning of Book VII of The Republic (514a-520a).
The cave allegory is probably best presented as a story, and then interpreted--as Plato himself does. Imagine prisoners chained since childhood deep inside a cave. Not only are their limbs immobilized by the chains, their heads are as well so that their eyes are fixed on a wall. Behind the prisoners is an enormous fire, and between the fire and the prisoners there is a raised way, along which men carry shapes of various animals, plants, and other things. The shapes cast shadows on the wall, which occupy the prisoners' attention. Also, when one of the shape-carriers speaks, an echo against the wall causes the prisoners to believe that the words come from the shadows. The prisoners engage in what appears to us to be a game--naming the shapes as they come by. This, however, is the only reality that they know, even though they are seeing merely shadows of images.
Suppose a prisoner is released and compelled to stand up and turn around. His eyes will be blinded by the firelight, and the shapes passing will appear less real than their shadows. Similarly, if he is dragged up out of the cave into the sunlight, his eyes will be so blinded that he will not be able to see anything. At first, he will be able to see darker shapes such as shadows, and only later brighter and brighter objects. The last object he would be able to see is the sun, which, in time, he would learn to see as that
- this it is that provides the seasons and the courses of the year and presides over all things in the visible region, and is in some sort the cause of all these things that they had seen. (The Republic bk. VII, 516b-c; trans. Paul Shorey)
This part of the allegory, incidentally, closely matches Plato's metaphor of the sun which occurs near the end of The Republic Book VI.