Allegory of the Cave. Imagine human beings living in an underground den, which has a mouth open towards the light. Here they have been from their childhood, and have their legs and necks chained so that they cannot move, and can only see before them, being prevented by the chains from turning their heads.
Above and behind them a fire is blazing at a distance, and between the fire and the prisoners there is a raised way; and you will see, if you look, a low wall built along the way, like the screen which puppet players have in front of them, over which they show the puppets.
To them, the truth would be literally nothing but the shadows of images. And now look again, and see what will happen when they are released and healed of their ignorance. At first, when any of them is liberated and compelled suddenly to stand up and turn his neck round, he will suffer sharp pains; the glare will distress him.