The philosopher Socrates wrote about a people who lived chained to the wall of a cave all of their lives, facing a blank wall.
The people watched shadows projected on the wall by things passing in front of a fire behind them.
They naturally gave names to these shadows, which were as close as the prisoners got to seeing reality.
He then explains how the philosopher is like a prisoner who is freed from the cave and comes to understand that the shadows on the wall do not make up reality at all, as he can perceive the true form of reality rather than the mere shadows seen by the prisoners.