Imagine some people that have lived in a cave their whole lives, chained on their stomachs so that their heads can only see the wall opposite the cave’s opening.
The only light that they see is that which comes from the entrance behind them projected onto the wall. The world as they know it is only shadows, but they think that these shadows are the real thing. They think that the shadow of a bird flying past the cave entrance is a real bird and so on.
One of the prisoners is released, his shackles removed, and he is allowed to turn around for the first time and see outside the cave. At first the light is blinding and he is afraid of it. He wants to stay back where he was in the comfort of the surroundings he knows and the reality he believes in. But eventually his curiosity gets the better of him and he ventures out of the cave.
For the first time, he sees reality for what it really is and he is a bundle of emotions. Overall, he is elated to see what a beautiful world exists beyond the cave. Every sense is heightened and he can see color and touch physical objects that he knew before only as silhouettes.
Excited and still concerned about his fellow companions, he runs back inside and tries to convince them that the shadows they see are not real; that reality lies just behind them and all they have to do is come with him and see. His statements are met with contempt and his sanity is questioned by the others.
– Allegory of the Cave can be found in Book VII of Plato’s best-known work, The Republic