Plato pictured prisoners chained in a cave since childhood, facing a wall.

All they ever see are shadows, thrown by a fire behind them.

Men carry figures along a low wall between the fire and the prisoners.

A shadow's size depends on its distance from the fire, not on the thing.

Carry the same figure one metre nearer the fire, and its shadow doubles.

They name the shadows, and take them for the real things.

Honours go to whoever best predicts which shadow comes next.

One is freed and dragged up into the sunlight. At first he sees nothing.

First shadows, then reflections in water, then the things themselves.

Then the moon and stars by night; last of all, the sun itself.

Back in the cave, his eyes full of darkness, he loses at judging shadows.

Up he went, they say, and down he came without his eyes.

Whoever tried to lead them up, they would put to death.