Plato's Allegory of the Cave is a symbol for the contrasts between ideas and what we perceive as reality. For example, Plato would argue that ideas transcend the physical world. Think of a cup. That cup could fall on the ground, it could crack, break... eventually it won't exist anymore. However, the idea of the cup will go on forever. The idea, once thought of, cannot be undone. It cannot be broken or tainted.