Plato
Plato believed in two worlds: the material world and the world of ideas. The material world is merely an imitation of the eternal world of ideas. Let me illustrate this. Surely no two horses are exactly similar, yet they all have something in common: they are horses. Plato believed that there must exist a perfect "idea" of a horse in the world of ideas, and all horses in the material world are imperfect copies of that ideal form. In this regard, Plato agreed with Heraclitus. He believed that everything in the material world flows and changes, but unlike Heraclitus, he thought there existed a permanent reality beyond it. While the material world is in constant flux, the world of ideas is eternal and unchanging. Plato explained this through his famous cave metaphor. Imagine three men born in a cave facing a wall. Behind them is a fire, and between the fire and the prisoners objects are moved, casting shadows onto the wall. Since the prisoners have never seen anything else, th...