fl. c. 500 B.C.

Greek philosopher to whom is attributed the doctrine that "all things are in flux and nothing is stable"—meaning the world is composed of opposites whose dynamic and constant tension gives rise to the apparent stability around us. Though often interpreted to mean everything is continually changing and therefore unknowable, Heraclitus did not deny the possibility of obtaining knowledge from sensory experience. On the contrary, he maintained that sensory experience, guided by proper understanding, was necessary to discovering the "Logos" that underlies and explains all things.