Heraclitus of Ephesus (Greek: Ἡράκλειτος ὁ Ἐφέσιος) was a pre-Socratic Greek philosopher, a native of the Greek city Ephesus, Ionia, on the coast of Asia Minor. He was of distinguished parentage. Little is known about his early life and education, but he regarded himself as self-taught and a pioneer of wisdom. From the lonely life he led, and still more from the riddling nature of his philosophy and his contempt for humankind in general, he was called "The Obscure" and the "Weeping Philosopher."

Heraclitus is famous for his insistence on ever-present change in the universe, as stated in his famous saying, "No man ever steps in the same river twice." He believed in the unity of opposites, stating that "the path up and down are one and the same," all existing entities being characterized by pairs of contrary properties. His cryptic utterance that "all entities come to be in accordance with this Logos" (literally, "word", "reason", or "account") has been the subject of numerous interpretations.