Ask any college student who Plato is, and if they have heard of him at all, they will undoubtedly also know the 'cave theory'. The Allegory of the Cave--as it is better known as--was written by Hellenic philosopher Plato in his work The Republic to illustrate 'our nature in its education and want of education'.
In The Republic, Plato has his mentor Socrates describe a group of people who have been chained to a wall in a cave their whole lives. In front of them is a blank wall. The only thing these people have seen all their lives lived are shadows, projected onto the wall by a fire burning behind the people. Everyone and everything that passes between the fire and the people is projected upon the wall. As humans do, the people begin to ascribe forms to these shadows, even though they can not see--and do not know--what or whom has caused these shadows. As the walls echo back sound, the people think that it is actually the shadows that speak, not the people behind them. When the people are released and look around, they will be shocked, horrified, and blinded by the light. They will fear what they see and for them, the shadows will be truer than everything around them, as they are used to the shadows.

Plato saw in these people the philosopher's struggle to look beyond the obvious, and look deeper into the fabric of the world, despite pain, fear, and shock. In Plato's words:

"...the prison-house is the world of sight, the light of the fire is the sun, and you will not misapprehend me if you interpret the journey upwards to be the ascent of the soul into the intellectual world according to my poor belief, which, at your desire, I have expressed whether rightly or wrongly God knows. But, whether true or false, my opinion is that in the world of knowledge the idea of good appears last of all, and is seen only with an effort; and, when seen, is also inferred to be the universal author of all things beautiful and right, parent of light and of the lord of light in this visible world, and the immediate source of reason and truth in the intellectual; and that this is the power upon which he who would act rationally, either in public or private life must have his eye fixed."

Many of us have heard this allegory in high school or college, but you've probably never heard it quite in the way Tim Wilson tells it in the following video, wherein he explains the entire analogy in great--and humorous--detail. If this theory was something you were struggling with, this video should fix that. You're welcome.

PS: I did not hear about this analogy in school, I heard it from a stoned friend while watching The Matrix. That movie, when you about it, is incredibly close to the allegory: Neo thinks he's living life, but in fact, he's trapped in a kind of shadow play. Once he yanked out violently, he is forced to learn about real life. After that, goes back in to free those still trapped inside. After that, he gets distracted, but that is besides the point. All of this to say that Plato's ideas are still very much relevant to this day.