Heraclitus (; Greek: Ἡράκλειτος Herákleitos, "Glory of Hera"; fl. c. 500 BC) was an ancient Greek pre-Socratic philosopher from the city of Ephesus, which was then part of the Persian Empire.

Little is known of Heraclitus's life. He wrote a single work, only fragments of which have survived. Most of the ancient stories about him are later said to be fabrications based on interpretations of the preserved fragments. His paradoxical philosophy and appreciation for wordplay and cryptic utterances has earned him the epithet "the obscure" since antiquity. He was considered a misanthrope who was subject to melancholia. Consequently, he became known as "the weeping philosopher" in contrast to the ancient philosopher Democritus, who was known as "the laughing philosopher".

The central idea of Heraclitus' philosophy is the unity of opposites. One of his most notable applications of this idea was to the concept of impermanence; he saw the world as constantly in flux, changing as it remained the same, which he expressed in the saying, "No man ever steps in the same river twice." This changing aspect of his philosophy is contrasted with that of the ancient philosopher Parmenides, who believed in "being" and in the static nature of the universe.