Plato, a student of Socrates, was one of the most influential thinkers in history. This lecture recounts his responses to Presocratics and his contemporaries. Key to understanding Plato and his scientific impact is his view of reality and how it affects the value he places on observation, the nature of true knowledge about the world, and how that knowledge is to be acquired. The influence of the secretive Pythagoreans is important both directly on Plato and through him, to the relationship between mathematics and the study of the natural world.