  A Greek philosopher of Ephesus (near modern Kuşadası, Turkey) who was active around 500 BCE, Heraclitus propounded a distinctive theory which he expressed in oracular language. He is best known for his doctrines that things are constantly changing (universal flux), that opposites coincide (unity of opposites), and that fire is the basic material of the world. The exact interpretation of these doctrines is controversial, as is the inference often drawn from this theory that in the world as Heraclitus conceives it contradictory propositions must be true.