Thales (c. 625-c. 545 B.C.)
Thales was a man of broad interests in science and mathematics. He likely traveled to Egypt to learn astronomy, geometry, and practical skills to do with the measuring and management of land and water. This practical knowledge enabled him to navigate ships. Also, according to the Greek historian Herodotus, he predicted an eclipse of the sun that occurred in 585 B.C.
Thales was different from his Egyptian predecessors because he tried to give a rational explanation of the world. He raised the question: What is the source of all things? His answer was “water.” He thought water was the cosmic stuff of the universe. All things come into being from water, and the earth floats on water like a log. But why did Thales think water was the ultimate substance? It's not known for sure. The writings of Thales have not survived. In fact, what is known of the Milesian philosophers is so fragmentary that philosophers must supplement it with the reports of Plato, Aristotle, and other philosophers and historians.
You can, however, make intelligent guesses about why he thought water was the primary substance. He might have chosen water because water can take many forms. It exists in all three forms of matter: solid, liquid, and gas. Aristotle thought it was more probable that Thales was influenced by the essential connection of moisture with life, as seen in such substances as semen, blood, and sap.
Thales' knowledge of geometry was practical. For one, he could measure pyramids in relation to the shadows they cast at certain times of the day. Also, the Greek historian Herodotus relates the story of how Thales once overcame the problem of getting an army across a river by diverting the flow of water to run behind the army's encampment until the channel in front of it was shallow enough to be crossed.
Like other Ionian philosophers, Thales believed in the doctrine of