Plato's Cave

Book 7 of the Republic opens with Socrates description of the 'cave'.  This is perhaps the most famous metaphor in all of philosophy.

Behold! human beings living in a underground den, which has a mouth open towards the light and reaching all along the den; here they have been from their childhood, and have their legs and necks chained so that they cannot move, and can only see before them, being prevented by the chains from turning round their heads. Above and behind them a fire is blazing at a distance, and between the fire and the prisoners there is a raised way; and you will see, if you look, a low wall built along the way, like the screen which marionette players have in front of them, over which they show puppets.
And do you see, I said, men passing along the wall carrying all sorts of vessels, and statues and figures of animals made of wood and stone and various materials, which appear over the wall? Some of them are talking, other appear silent.
Like ourselves...they see only their own shadows of one another, which the fire throws on the opposite wall of the cave? ... And of the objects which are be carried in like manner they would only see the shadows? ...  And if they were able to converse with one another, would not suppose they there were naming what was actually before them? ... And suppose further that the prison had an echo which came from the other side, would they not be sure to fancy when one of the passers-by spoke that the voice which they heard came from the passing shadow?
...
To them, I said, the truth would be literally nothing but the shadows of the images.  They couldn't know the true world.  Or rather, they could only get 

So everything that the chained humans would see and know would be the distorted images of shadows.  They couldn't know the subtle truths of the real world.  Socrates continues:

And now look again, and see what will naturally follow if the prisoners are released and disabused of their error. At first, when any of them is liberated and compelled suddenly to stand up and turn his neck round and walk and look towards the light, he will suffer sharp pains; the glare will distress him, and he will be unable to realities of which in his former state he had seen the shadows; and then conceive some one saying to him, that what what he saw before was an illusion, but that now, when his approaching nearer to being and his eye is turned towards more real existence, he has a clearer vision, -what will be his reply? And you may further imagine that his instructor is pointing to the objects as they pass and requiring him to name them, - will he not be perplexed? Will he not fancy that the shadows which he formerly saw are truer than the objects which are now shown to him?

This is the philosopher.  He is released from the bonds of normal humanity and led into the true light.  It is painful and he is uncertain but he can actually see the Truth.  Socrates goes on to talk about how it would take time for the eyes to adjust and how badly the eyes would work if this person is led back underground.  After spending time in the true sun of philosophy, he would have trouble working in the normal conditions of everyday life, i.e. the cave.
This philosopher would also have a hard time convincing the prisoners of the truth.  They would distrust him and doubt his stories.  They would take his dim vision as proof that he had followed a dangerous path.  And the more radical his stories seemed, the more they would doubt.

The total idea here is enormous.  Instead of risking turning this post into something monstrous, I'm going to split it in two.