Thales, the earliest Greek philosopher of nature (physis), said that all the world is made of water. He thought that this was its ultimate substance, that underlying all the appearances of variety and change, this was the common ground, the essence of all things.
Thales was wrong, of course, but he started the enterprise of philosophy going in a new direction: seeking the essential nature of the real world in matter, without reference to invisible qualities, miracles or spiritual forces, as had previously been done.
Following Thales came other philosophers who identified the ultimate substance differently, such as fire or air, but they all identified the ultimate substance in terms of something in the natural world.
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