Heraclitus was a Greek philosopher who lived around 535-475 BCE. He is known for his doctrine of change and the concept of the unity of opposites, famously stated as "You cannot step into the same river twice." He believed that change was the fundamental nature of reality and that everything was in a state of flux. He also believed that the universe was governed by a rational order, which he called the Logos, and that the key to understanding the world was to understand the underlying principles of change.

Heraclitus wrote a book, On Nature, which has not survived in its entirety, but his ideas are referenced in other texts and through fragmentary quotes. He was known for his enigmatic and paradoxical statements, which have led to many different interpretations of his philosophy.

Heraclitus' ideas had a significant influence on later philosophers, particularly the Stoics. His emphasis on the importance of change and the unity of opposites is considered a fundamental aspect of Western philosophy, and his concept of the Logos has been interpreted in various ways, from the rational order of the universe to the word of God. He was also considered as one of the founders of the pre-Socratic philosophy, and his ideas were studied by philosophers for centuries.