Heraclitus was an ancient Greek philosopher who lived in the 6th century BC. He is best known for his philosophy of change being central to the universe, as expressed in the famous saying, "No man ever steps in the same river twice." He believed that change was the only constant in life, and that all things are in a state of flux. He also believed that the universe was composed of a single, unified substance, which he called Logos. He was an influential figure in the development of Western philosophy, and his ideas have had a lasting impact on the way we think about the world.