Heraclitus was an ancient Greek philosopher known for his belief that change is the fundamental essence of the universe. He famously stated that "you cannot step into the same river twice," highlighting his view that everything is in constant flux. This idea suggests that life is always evolving, and nothing remains the same for long. Heraclitus also introduced the concept of Logos, which refers to the underlying order and reason in the universe. He believed that opposites are interconnected and that conflict is necessary for harmony. His thoughts laid the groundwork for later philosophical discussions about the nature of reality and existence.