Thales (c. 624–c. 547 BC)

Greek philosopher and scientist. He made advances in geometry, predicted an eclipse of the Sun in 585 BC, and, as a philosophical materialist, theorized that water was the first principle of all things. He speculated that the Earth floated on water, and so proposed an explanation for earthquakes. He lived in Miletus in Asia Minor.

Thales explained such events as earthquakes in terms of natural phenomena, rather than in the usual terms of activity by the gods. Aristotle records that, when he was reproached for being impractical, Thales, having predicted that weather conditions the next year would be conducive to a large olive harvest, bought up all the olive presses in Miletus and exploited his monopoly to make a large profit.

In five fundamental propositions Thales laid down the foundations on which classical geometry was raised. (1) A circle is bisected by its diameter. (2) In an isosceles triangle the two angles opposite the equal sides are themselves equal to each other. (3) When two straight lines intersect, four angles are produced, the opposite ones being equal. (4) The angle in a semicircle is a right angle. (5) Two triangles are congruent if they have two angles and one side that are respectively equal to each other. He is also said to have introduced the notion of proof by the deductive method.