Presocratic Philosophers: Start

According to the Internet Encyclopedia of Philosophy, "Presocratic philosophers are the Western thinkers preceding Socrates (c.469-c.399 B.C.E.) but including some thinkers who were roughly contemporary with Socrates, such as Protagoras (c.490-c.420 B.C.E.). The application of the term “philosophy” to the Presocratics is somewhat anachronistic, but is certainly different from how many people currently think of philosophy. The Presocratics were interested in a wide variety of topics, especially in what we now think of as natural science rather than philosophy. These early thinkers often sought naturalistic explanations and causes for physical phenomena...Such an emphasis on physical explanations marked a break with more traditional ways of thinking that indicated the gods as primary causes." 

Picture Credit:

Greek column: http://www.bible-history.com/ibh/images/fullsized/column-Corinthian-style-one-of-the-columns-at-Temple-of-Olympian-Zeus-in-Athens.jpg