Several Greek philosophers before the time of Socrates (d. 399 B.C.) grappled with issues that laid the foundations of Western natural philosophical thought and method: What is the world made of? Where did things come from? Do our senses show us reality? We study their explanations for the physical changes around us, their ideas on the origin and end of the world, and the new concept of atoms. We consider how the influence of Presocratic ideas has resounded in Western thought ever since.