Heraclitus (c. 500 BC), a pre-Socratic Greek philosopher from Ephesus, is renowned for his investigation of change, the unity of opposites, and the concept of logos. His surviving fragments, compiled in On Nature and Fragments of Heraclitus, highlight flux, as seen in his famous assertion "Everything flows" (πάντα ῥεῖ).
Heraclitus regarded fire as the fundamental substance of the universe, claiming that all things undergo constant transformation. His enigmatic style and focus on paradox have had a lasting influence on Western philosophy, from Plato to Nietzsche, emphasizing his continued significance in discussions on existence and evolution.