The Greek philosopher Heraclitus (ca. 540-ca. 480 BC) attempted to explain the nature of the universe by assuming the existence of the logos, that is, order or reason, as the unifying principle which guides all things and by specifying fire as the basic substance which underlies physical reality.

Heraclitus was born in the lonian city of Ephesus and is said to have renounced the privileges to which his social rank entitled him (perhaps the kingship) in favor of his brother. The available evidence for his life is too scanty for a clear picture to emerge. He is a solitary figure who claims to have sought the truth within himself, and although his work shows familiarity with the writings of other philosophers, particularly those of Anaximander, both his unique ideas and his peculiar literary style set him apart.

Many fragments of Heraclitus's work, commonly known as On the Nature of...