Heraclitus appears to believe that underlying all change was an eternal principle, logos, that did not change.

Heraclitus

An ancient Greek philosopher known for his doctrine that change is the fundamental essence of the universe, famously encapsulated in the phrase "You cannot step into the same river twice."

Eternal Principle

An unchanging truth or law that is considered fundamental to the nature of reality.

Logos

(from the Greek logos.) Word, reason, or logic. The term is sometimes used to refer to the eternal principle underlying reality, giving the world order, or to the divine intelligence that is manifest in this order.