An Ionian, the founder of Greek philosophy and the Milesian school of cosmologists. Born at Miletus about 636 B.C., he was a contemporary of Solon and Croesus and died at the age of ninety about 546. According to popular tradition he lived mainly as one of the Seven Sages -- seven men living between 620-550 B.C. distinguished for their wisdom as rulers, lawgivers, councilors, or authors of maxims. Many tales were told of him. In one of these he is an unpractical dreamer, and falls into a well while star-gazing. In another he shows himself superior to the ordinary practical man by the use he makes of his scientific knowledge. He is said to have foreseen an abundance of olives and made a corner in oil, thus proving he could be rich if he liked. It is plain that people in general had no idea of his real work, and regarded him simply as a typical 'sage', to whose name anecdotes originally anonymous might be attached. These stories, then, tell us nothing about Thales himself, but they do bear witness to the impression produced by science and scientific men when they first appeared in a world that was half inclined to marvel and half inclined to scoff.
There is, however, another set of traditions about Thales from which something may be learnt. They are not of a popular character, since they attribute to him certain definite scientific achievements. One of the most important of these, the prediction of a solar eclipse, is reported by Herodotus. The existence at Miletos of a continuous school of cosmologists give credit to the preservation of such traditions. We are further told on the authority of Aristotle's disciple Eudomos, who wrote the first history of mathematics, that Thales introduced geometry into Hellas. However, the evidence is incomplete since no writings by Thales were extant even in the time of Aristotle, and it is believed that he wrote nothing. In the annals of Greek philosophy he was probably the first who looked for a physical origin of the world instead of resting upon mythology. As such he is credited with being the first human being who can rightly be called a man of science.
His philosophy, if it can be called by that name, consisted of two propositions. The first is that the earth is a flat disc which floats on water. From an early date the Greeks, as was natural for them, began to think of the earth as an island surrounded by the river Okeanos. To regard it as resting on the water is a further step towards a truer view. It was something to get the earth afloat. The second is the principle of all things is water, that all comes from water, and to water all returns. This means that water is the one primal kind of existence and that everything else in the universe is merely a modification of water. In Aristotle's terminology, Thales maintained that water is the material cause of all things. It may well have seemed to Thales that water was the original thing from which fire on the one hand and earth on the other arose. Henceforth the question whether everything can be regarded as a single reality appearing in different forms is the central one of Greek science. Why did he choose water as the first principle? This question cannot be answered with certainty. Aristotle says that Thales "probably derived his opinion from observing that the nutriment of all things is moist, and that even actual heat is generated therefrom, and that animal life is sustained by water... and from the fact that the sees of all things possess a moist nature, and that water is a first principle of all things that are humid." This is likely the true explanation, but it will be noted that even Aristotle uses the word "probably," so gives his statement merely as a conjecture. It is even more uncertain by what process water changes into other things.
Excerpt from the Encyclopedia Britannica without permission.