Heraclitus of Ephesus was a Greek philosopher active around 500 BCE, in the late 6th to early 5th centuries BCE. He was born into an aristocratic family in the Ionian city of Ephesus. According to later stories, he was known for a reclusive and critical temperament, which earned him ancient nicknames like "the Obscure" and "the Weeping Philosopher."

He is traditionally credited with a single major prose work, often referred to by later authors as On Nature, though its original title is unknown. This book does not survive intact. Instead, about 130 short, often paradoxical statements from it are preserved as fragments in the writings of later philosophers, historians, and Christian authors.

Heraclitus is a foundational figure in early Greek philosophy. His significance lies in a system built on the idea of constant change, famously expressed in the concept that "everything flows" and the analogy that one cannot step into the same river twice. He proposed fire as a primary cosmic principle and introduced the idea of the Logos, a rational order or law governing the universe's changes. His work emphasized the unity of opposites and a hidden harmony within apparent conflict. According to modern scholars, his cryptic style and ideas deeply influenced later thinkers, including Plato, the Stoics, and many philosophers into the modern era.