There was a cavern. Several men inside chained
from their heads and feet so that they could only
look at the front wall where shadows of objects
moved by other men up behind them and a fire light which
produce the shadows
 on the front wall in front of chained men.
One of them is freed and taken outside to know
other realities available in world.
He returned to the cavern after this discovery so as to free
his companions but they think he's
crazy. They neither believe him nor want to
move from the known place.

brief tale of Cavern Myth or Alegory
in" Republic", Book VII, Plato.