Heraclitus of Ephesus was a Greek philosopher active around 500 BCE. He was a member of an aristocratic family in the Ionian city of Ephesus. Later tradition gave him the nicknames "the Obscure" and "the Riddler" due to the paradoxical and enigmatic style of his writing.

He is credited with a single book, often referred to by later authors as On Nature. The original work does not survive. Modern knowledge comes from approximately 130 short, aphoristic fragments preserved in the writings of later philosophers and Christian authors.

Heraclitus is a foundational figure in early Greek philosophy. His central concept is the Logos, an underlying principle of order that governs the cosmos. He is most famous for the doctrine of universal flux, often summarized by the sayings "everything flows" and "you cannot step into the same river twice." According to modern scholars, he viewed the cosmos as an ever-living fire undergoing constant transformation, and he emphasized the unity of opposites. His obscure style and profound ideas significantly influenced later thinkers, including Plato and the Stoics.