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Imagine human beings living in an underground, cave like dwelling, with an entrance a long way up, which is both open to the light and as wide as the cave itself They've been there since childhood, fixed in the same place, with their necks and legs fettered, able to see only in front of them, because their bonds prevent them from taming their heads around. Light is provided by a fire burning far above and behind them. Also behind them, but on higher ground, there is a path stretching between them and the fire. Imagine that along this path a low wall has been built, like the screen in front of puppeteers above which they show their puppets... Then also imagine that there are people along the wall, carrying all kinds of artifacts that project above it -- statues of people and other animals, made out of stone, wood and every material ... Then the prisoners would in every way believe that the truth is nothing other than the shadows of those artifacts."

-- Plato, "Republic, in Classics of Moral and Political Theory," --