Heraclitus was a pre-Socratic philosopher who, along with his polar opposite, Parmenides, set out the basic premises of Western thought. While Parmenides emphasized the strict immutability of existence, Heraclitus argued for the continuous alteration of all that exists. Much of later Greek philosophy can be viewed either as an acceptance of one position and rejection of the other or as an attempt to reconcile the two. In his conception of the cosmos Heraclitus looks back to the beliefs of earlier eastern Greek thinkers (often called Milesian, after the city of Miletus) who, less consciously and explicitly than he, were proponents of the doctrine of change; he also foreshadows the work of Plato, Aristotle, and others, who, quite consciously, felt the need to incorporate Heraclitean flux into their cosmologies and epistemologies.

Little about Heraclitus's life is known for certain. All sources agree that he was born and lived in...