Thales, the first Western philosopher, said everything is made of water

Around 600 BCE, Thales of Miletus broke from myth and proposed the first purely naturalistic theory of the cosmos: that a single material principle, water, underlies and generates everything that exists. It sounds crude today, but treating nature as explicable by nature itself, no gods required, is why historians call Thales the first philosopher.

— Aristotle, Metaphysics (testimony on Thales) — Metaphysics, Book I, 4th c. BCE