The Allegory of the Cave - Plato's "Republic"
In Plato's Allegory of the Cave we begin to get an understanding of the innovation that he produced in response to the central question of the Pre-Socratics, "What is the cause of the change and identity of the things in nature?". Plato's answer is that the change that goes on in nature happens as it is the physical world of the senses, while the identity that remains in things is because of its intellectual nature, like those of ideas. To illustrate this, only think of a tree outside your window. You see its green leaves and branches. Then look to another tree nearby. It looks different in its positioning and make up. But yet you know that both are "trees". It is the mind which sees the identity in things on an abstract level while the senses see the physical changes that are always taking place. Therefore, it must be that the physical world is produced by a higher intellectual world, of which we can only have glances and brief grasps. The role of the philosopher, Plato makes clear, is then to cultivate the true life of the mind and to shun the passing nature of the body. Now to get to Plato's allegory.
In the allegory there is a cave in which prisoners have been kept since birth. They have been chained against the back wall and not allowed to look anywhere but that back wall for their whole lives. Behind them is a low wall and a walk way in which people can pass by with puppets. Behind that is a fire that reflects light over the wall and casts shadows for the prisoners of the puppets. So the prisoners grow up their whole life thinking that reality is the shadows they see on the wall of the different images.
Plato then speculates if a prisoner was freed from his chains. He gets up and proceeds back towards the fire, from which he is blinded at first because of its brightness. Then he's confused as he sees the puppets and realizes that they are responsible for the shadows on the wall. Even further, he is pulled out of the cave and into the daylight above. There he is really blinded by the brightness of the sun and can only perceive shadows at first. Eventually his eyes adjust and he is able to make out figures, and ultimately the sun itself which is the source of all his knowledge through its light.
Having come to realize that there is a more real world than the one he grew up with, he feels pity on his fellow prisoners and ventures to go back and free them. The problem is that once he goes back into the cave he cannot see very well and appears odd to the others. The prisoners, Plato says, have created awards and honors for each other based on their knowledge of the shadows. The enlightened man appears foolish as he tries to convince them that there is something more real than the shadows. They don't not want to listen or to leave. In fact, they even try to kill the enlightened man for the message that he is bringing to them.
Meaning of the Allegory
"This entire allegory, Glaucon, you may now connect with the previous argument. The cave is the world of sight, the light of the fire is the sun. You will not misunderstand me if you interpret the journey upwards to be the ascent of the soul into the intellectual world."
The philosopher then too will look foolish in society as he has been enlightened to the importance of the intellectual life while others have not.
"Is there anything surprising if someone acts in a strange manner if he passes from divine contemplation to the evil state of human existence? While his eyes are blinking and before he has become accustomed to the surrounding darkness, he may be compelled to fight in courts of law, or in other places, about the images or the shadows of images of justice. He would be trying to address the conceptions of those who have never yet seen absolute justice."
Republic, VII 514 a, 2 to 517 a, 7