-535 – -475
Heraclitus of Ephesus (c. 535–475 BCE) was a pre-Socratic Greek philosopher known for his doctrine that change is the fundamental essence of the universe, encapsulated in the saying 'everything flows' (panta rhei). He posited the Logos—a rational principle governing all things—and argued that apparent opposites are unified through dynamic tension. His cryptic, aphoristic style earned him the epithet 'the Obscure' among ancient commentators.
Developed the doctrine of universal flux: reality is defined by constant change and becoming
Introduced the concept of the Logos as the rational principle underlying all natural processes
Articulated the unity of opposites: contraries like hot/cold and day/night are aspects of a single underlying reality
Proposed fire as the primary arche (fundamental substance) governing cosmic cycles
Influenced Stoic cosmology and philosophy of nature, especially their concept of the Logos