The past is not a story; only in retrospect under an interpretation does it unfold as history like a fictional tale in a book. Consequently, in reporting what happened in the past we lack one of the characteristics of a story: a definite beginning. However, in Greece a short time after 600 bc certain changes were taking place in human thought that seemed to have no precedent; and it is on these changes in the way human beings began to think about the world and themselves that the most fundamental aspects of today's Western civilization – its science, ethics, politics, and philosophy – are founded. There were events of significance before this time; but 600 bc onwards marks alterations in human thought sufficient to describe it as a beginning.

The study of ancient philosophy is normally said to extend from 585 bc to ad 529. Of course, philosophical speculation did not cease at that date, but the banning of the teaching of Greek philosophy at the University of Athens by the Roman Christian Emperor Justinian, in ad 529, is thought of as a suitable event to mark a change.

The Presocratic period covers 585 bc to 400 bc and the term “Presocratic” has the obvious literal sense of denoting those philosophers living before Socrates. This meaning is only approximate, as some of the philosophers considered as Presocratics were contemporaries of Socrates who was born in 470 bc and died in 399 bc.