"Picture the following in your mind. Imagine human beings living in an underground cave-like residence. Its entrance opens up to the light and reaches all along the cave. They have been there since their childhood, their ankles and necks chained, unable to move or turn their heads, forced to look ahead. The light from a fire blazing at a distance comes from above and behind them. Between the fire and the prisoners there is a raised walkway. Imagine also a low wall built along the way, similar to the screen that divides puppeteers from the audience and allows them to show puppets over it.
...
Now imagine that people walk behind the wall and carry various artifacts that extend above the wall. These artifacts include carvings of humans and other animals made of stone, wood, and other materials. Some of the people carrying these object are talking, while others are silent. ...They are like ourselves. Now do you think they see anything else except their own shadows, or the shadows of one another, which light from the fire casts on to the opposite wall of the cave?
...
Now imagine what would happen naturally if the prisoners were released from their shackles and cured of their ignorance. Right after they are released and suddenly forced to stand up, turn their necks around, walk, and look towards the light, these activities will cause them pain; because of the bright glare they would be unable to see those things which they previously had seen only as shadows. Now what do you think they would say if one were to tell them that what they saw before was fooling them, but that now, when they are closer to what really exists and when they face that which more truly exists, they see more clearly, in a straightforward manner? What if that person pointed to the objects as they passed and asked the former prisoners to tell him what they were? Don’t you think they would be baffled and think that the shadows they formerly saw were truer than the objects that are now being pointed out to them? "
- Plato (c.424 - 348 BC)
"The Allegory of the Cave" (Republic, Book Seven)


