Aristotle called him the "originator of this kind of philosophy." He held water to be the principle of all natural things.
Thales of Miletus (c. 624/623 - c. 548/545 BC) held water to be the principle of all natural things. This was his most famous philosophical position. This cosmological thesis of Thales comes down to us from Aristotle's Metaphysics. Aristotle reports it as Thales' hypothesis about the nature of all matter. The originating principle of nature was a single material substance: water.
Aristotle did not hold this view himself, but listed some reasons as to why Thales probably thought it.
Thales even pictured the Earth as a flat disk that was sitting in some kind of expanse of water. St. Thomas Aquinas says that "anything having a principle is based on its principle. "
Thales was an Ioanian philosopher, a group who Aristotle calls "those who discourse on nature."