In The Republic, Plato likens the majority of humans to prisoners in a cave. The only light in this cave comes from a fire which casts shadows of various puppets onto the walls. The prisoners are in chains, cannot turn their heads and have been in this situation since childhood. Since the cave is the only reality they have ever known, they believe that what they see and hear simply is reality: “the truth would be nothing but the shadows of the images.”

What Plato means is that when we look at the world, we do not see reality itself. The objects that we see around us are like the shadows on the wall since they are just poor imitations of reality. When we see a chair, for example, we see something which functions well enough as a chair, but any physical chair in the world will fall short of what Plato takes to be the idea of a chair, or to put it another way, the ideal chair.

This relates to Plato’s theory of Forms. Plato believes that anything we see in the world partakes of a Form. And just as a painting of a chair is less real than a physical chair, so a physical chair is less real than the Form of a chair. And while a physical chair depends on the Form of a chair for its existence, the Form of the chair does not depend on anything at all.

The job of the philosopher is to leave the cave of shadow objects and through philosophical study, go out into the light of the world of Forms, where reality is to be found. The most important form of all, the Form of the good must be uncovered and its truth must be brought back to the other prisoners in the cave.