Experience?
Plato placed knowledge in dettaching oneself from the illusions of material existence and looking for the truth in the world of Ideas. He believed that the material world was nothing but a distorted shadow of the original and authentic reallity. Truth, beauty and good exist in things in the same degree that they are near the immaterial, divine essence of ideas.
Aristotle was convinced that we can only truly know what our senses can reach. The only the material world could be grasped, only it could be subject to analysis, and true or false is ultimately based on the coherence of the data from experience.