Thales of Miletus

Thales was born around 624 BC in Miletus, a Greek city on the coast of Ionia (today's western Turkey), then the busiest port of the Greek world, trading with Egypt, Lydia and the Black Sea. Nothing he wrote survives, if he wrote anything at all, and everything we know comes from later authors: Herodotus a century after him, then Plato and Aristotle and, much later, the anecdote collector Diogenes Laertius. Even his dates are reconstructed from the one event that can be fixed, the solar eclipse of 28 May 585 BC, which he is said to have predicted.

He seems to have travelled to Egypt, where the Greeks believed he learned geometry from the priests, and he was active in the politics of Ionia: Herodotus says he advised the Ionian cities to unite in a single council at Teos to resist the rising power of Lydia and Persia, advice they did not take. In antiquity he was counted first among the Seven Sages, the wise men of early Greece, and the stories about him pull in two directions. In one, told by Plato, he falls into a well while gazing at the stars, and a Thracian servant girl laughs at the man who wants to know what is in the sky but cannot see what is at his feet. In another, told by Aristotle, he foresees a good olive harvest, rents every olive press in Miletus and Chios cheaply during the winter, and makes a fortune when the harvest comes, to prove that philosophers could be rich if they cared to be. He is said to have died at a great age, around 546 BC.

Aristotle calls Thales the founder of natural philosophy because he asked a new kind of question: not which god made the world, but what the world is made of. His answer was water, the ἀρχή (arche, first principle) of everything, perhaps because the earth seemed to float on it, perhaps because every living thing needs it. The answer matters less than the move: a single natural substance, open to argument, instead of a myth. His successors in Miletus, Anaximander and Anaximenes, immediately disagreed with him, and that habit of disagreement is the beginning of science.

In mathematics he is credited with the first geometrical proofs. The Θεώρημα του Θαλή (Thales' theorem) taught in Greek schools says that parallel lines cutting two other lines divide them into proportional segments; the theorem named after him abroad states that an angle inscribed in a semicircle is a right angle. With the idea of proportion he is said to have measured the height of the Egyptian pyramids, comparing their shadow with the shadow of a stick, and the distance of ships at sea from the shore. In astronomy he was credited with predicting the eclipse of 585 BC, which stopped a battle between the Lydians and the Medes, and with showing sailors how to steer by the Little Bear. How much of this is true is debated; what is not debated is that for the Greeks who came after him, from Herodotus to Aristotle, Thales was the first man to think in this way.

In Greece Thales lives chiefly in the classroom. Το θεώρημα του Θαλή is a fixed point of the lower-secondary maths syllabus, so every Greek adult remembers the name even after forgetting the theorem, and the lyceum philosophy textbook opens with him and the water. The Hellenic Mathematical Society's national competition for school pupils is called «Θαλής» and is held every autumn, with the later rounds named after Euclid and Archimedes. The story of the philosopher who fell into the well is still told as a gentle joke about absent-minded academics, and «Θαλής» is a common first name. Miletus itself, with its great theatre and a harbour long since silted up by the river Maeander, can be visited near Didim in Turkey, a few kilometres inland from the sea that once made Thales' city rich.

• Not a single line of his survives; everything we know comes from writers who lived one to eight centuries later.
• The eclipse he is said to have predicted, on 28 May 585 BC, is one of the earliest events in Greek history that can be dated to the day.
• According to Aristotle, he cornered the market in olive presses one winter to prove that a philosopher could get rich if he wanted to.
• Greece's biggest school maths competition, run by the Hellenic Mathematical Society, is named «Θαλής» after him.