Ephesus was an important philosophical center in antiquity, and its most renowned philosopher was Heraclitus, who lived around 540–480 BCE. Heraclitus proposed that fire was the fundamental substance of the universe and argued that everything is in a state of constant change. This idea is encapsulated in the phrase “Panta Rhei” (“Everything flows”), emphasizing that everything in the universe is in perpetual flux and that nothing remains fixed. According to Heraclitus, the universe consists of harmony and the unity of opposites, with logos (reason or logic) playing a central role in maintaining this dynamic balance.