Plato's Allegory of the Cave comes from Book VII of the Republic, one of his longer books, in which he sets out his vision of the ideal society. One of his ideas is that society should be run by philosophers, because (obviously) they're the ones who understand what the world is really like, so Book VII is part of his explanation of what it means to be a philosopher. The allegory goes as follows:
Socrates: Imagine a cave, with people sitting in it. Light comes in at the mouth of the cave, and towards the end of the cave, some people are sitting, chained up by their legs and their necks, so they cannot move and can only look towards the back of the cave. There is a low wall behind them, and behind the wall a fire. Behind the wall, people are moving around, sometimes speaking, sometimes silent, carrying different objects around like puppeteers. Their movements make shadows on the wall in front of the prisoners.
Glaucon: That's kind of weird.
Socrates: Well, Glaucon, life is kind of weird. Now, these prisoners can't see anything of what's going on behind them, only the shadows that are cast on the wall they face, because they are chained in such a way that they can't look round. And if they started talking to each other, they wouldn't realise they were talking about shadows, they'd think they were talking about
the real things. Now, imagine that one of the prisoners was unchained, and could turn around and see the light. Their eyes wouldn't be used to such brightness, and they would be dazzled. If someone told them now that what they were seeing was reality, and what they saw before was just shadows, they wouldn't believe you.
Now, imagine that someone dragged them out of the cave, into the light of the sun itself. For a person who'd lived in a cave their whole life, this would be even more painful for the eyes, and they'd only slowly start being able to see anything. First they'd be able to see the things the sun's light lit up, and eventually they'd be able to look at the sun itself (was the sun darker in ye olde days, or their eyes more adaptable? Hey ho). Finally, able to see the sun, they would come to believe that the sun was the cause of the seasons and the years, the source of all the light in the world.
When this person remembered all the other people still locked up in the cave, they might start feeling sorry for them. Thinking about their attempts to decipher the shadows, they would feel pity, and consider all the honours they bestowed on each other pretty unimportant. But if this person went back into the dark, they'd struggle to see the shadows, and the others might think they were stupid, and consider their movement around the cave dangerous.
This is what the world is like. The cave is the world of sight, the fire in the cave is the sun. The journey out of the cave is the ascent of the soul to the intellectual world. The sun is the idea of the Good, the source of everything beautiful and good in the world. Can you see that it is necessary to see the Good in order to be able to make good decisions about how to act? And can you understand that people who have achieved the vision of the Good might be reluctant to get involved with petty human affairs? Can you see that the people stuck in the world of sight might not understand the insights of the philosophers? Can you see that in adjusting to the real world or to the cave, philosophers might go through periods of readjustment, and might seem a bit dim for a while? Can you see it?
Glaucon: Yes, Socrates, I can. Thanks for letting me contribute to this dialogue.
Lovely story, no? Here's the thing: while it's nice to know about Plato so you can sound cultured, it's also pretty important for understanding a lot of theology. Until very recently, the study of Greek philosophical thought was part of the education of pretty much everyone in the Western world who got an education, including most of the people who wrote theology. This story is everywhere in theology. You see it particularly in discussion of what it means to know God, who is, predictably, usually taken to be the idea of the Good.
It also has a few consequences which I'm particularly interested in right now: firstly, it makes the physical world less real, something that might start us off in the direction of knowledge of God, but will, in the end, get in the way of seeing the truth about reality. It makes our bodies ultimately a hindrance to seeing God. As a result, it makes social and political engagement at best an unfortunate necessity: no one who has seen God as he really is will really want to be mixed up in the messy business of this world - instead, they'll want to be off contemplating God. It also creates elitism: mostly people don't really understand the way the world really is, poor things, and until they too become contemplatives, they won't get it. Except, oops, only a tiny minority of people in the world get to become monks, and only monks get to really devote themselves to contemplation. This gets interesting when people start using theologians like Thomas Aquinas, who totally buys into this account of the world, to talk about contemporary Christian ethics which people do, well, a lot, even secular thinkers who are interested in his ideas, without really taking into account their underlying vision of the way things really are.
And if that wasn't enough (philosophy, patristics and ethics all at the same time!), you can go watch the award winning live action version.
Photo credit: 智熏 on Flickr