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[thA' lEz uv mI lE' tus] Ancient Greek philosopher and a founder of geometry and abstract astronomy
Thales was born sometime around 640 B.C., probably in Miletus, Asia Minor (now Turkey), and died about 546 B.C. He probably engaged in some form of commerce, as Miletus was at the time a major trading port, but to what extent is not known. It is assumed that, like other learned men of his day, he made at least one trip to the fabled libraries of Egypt, if not several. As for the specifics of his life, however, nothing is truly known. Most of the information about his life and work comes from accounts written after his lifetime, and many of those accounts are themselves taken from other writings.
Thales's philosophy is usually summed up in the dogma "all things are water." Like many of his contemporaries, Thales searched for a single material cause for all things, one completely independent of the whims of gods and goddesses. By observing the part which moisture plays in the production and the maintenance of life, Thales determined that that material was water, and that all substances therefore come from water and will eventually revert to water. How water came to become any specific thing, however, Thales does not appear to have concerned himself with.
An extension of Thales's basic belief was that the earth was a spherical body that rests on water, floating much like a piece of wood floats in a pond. And, since all events occur because of the action of water, it was his belief that earthquakes occur when that water is agitated. As with the transformation of water into a specific form, what caused the water's agitation was not of concern to Thales.
His Scientific Work
Although the specifics of Thales's life and philosophy are subject to disagreement, the advances he made in the field of mathematics are not. The following discoveries in geometry are attributed to Thales: (1) the circle is bisected by its diameter; (2) the angles at the base of an isosceles triangle are equal; (3) when two straight lines intersect the vertically opposite angles are equal; and (4) the angle in a semicircle is a right angle. Two applications of these discoveries to practical problems are also attributed to him: (1) the determination of the distance of a ship at sea; and (2) the determination of the height of a pyramid by means of the length of its shadow -- the shadow was measured at the hour of the day when a man's shaow is the same length as himself.
Thales is also said to have been a careful observer of the heavens, specifically of the movements of heavenly bodies. Anecdotal accounts suggest that he was the first to call the last day of the month the thirtieth, to divide the year into 365 days, and to realize that the length of time between the solstices is not always uniform. It is also said that he instructed his countrymen to steer by the constellation of Ursa Minor (Little Bear, Little Dipper) instead of Ursa Minor (Big Bear, Big Dipper), because Ursa Minor has a smaller orbit in relation to the North Pole and is, therefore, a much more reliable celestial guide. Lastly, it was said that Thales predicted the total eclipse of the sun which occurred on May 28, 585 B.C., which so disrupted an ongoing battle between the Medes and the Lydians that it caused a cessation of hostilities and led to a lasting peace between the two parties.
The doctrine of Thales was interpreted and developed by Anaximander, Anaximenes, Heraclitus, and others.
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This page was last updated on June 21, 2017.