The Milesians

 Pythagoras' life is lost in legend. Rather more is known about a group of philo­sophers, roughly contemporary with him, who lived in the city of Miletus in Ionia, or Greek Asia. The first of these was Thales, who was old enough to have foretold an eclipse in 585. Like Pythagoras, he was a geometer, though he is credited with rather simpler theorems, such as the one that a circle is bisected by its diameter. Like Pythagoras, he mingled geometry with religion: when he discovered how to inscribe a right-angled triangle inside a circle, he sacrificed an ox to the gods. But his geometry had a practical side: he was able to measure the height of the pyramids by measuring their shadows. He was also interested in astronomy: he identified the constellation of the little bear, and pointed out its use in navigation. He was, we are told, the first Greek to fix the length of the year as 365 days, and he made estimates of the sizes of the sun and moon.

Thales was perhaps the first philosopher to ask questions about the structure and nature of the cosmos as a whole. He maintained that the earth rests on water, like a log floating in a stream. (Aristotle asked, later: what does the water rest on?) But earth and its inhabitants did not just rest on water: in some sense, so Thales believed, they were all made out of water. Even in antiquity, people could only conjecture the grounds for this belief: was it because all animals and plants need water, or because the seeds of everything are moist?