Thales of Miletus

Thales of Miletus was a pre-Socratic Greek philosopher from the Ionian city of Miletus in Asia Minor. He lived sometime in the late 600s and early 500s BC and was one of the Seven Sages, key founding figures of ancient Greece. Starting in the 1700s, historians began to see him as the first Greek philosopher, since he broke with mythology to explain the world through natural philosophy. For that reason, he is also considered the first to practice mathematics, science, and deductive reasoning.

Thales thought all of nature came from a single basic substance, which he identified as water. He also believed the Earth floated on water. In mathematics, two theorems are named after him: Thales's theorem and the intercept theorem (which is also sometimes called Thales's theorem). He is said to have figured out the heights of the pyramids and how far ships were from shore. As an astronomer, he supposedly predicted the weather and a solar eclipse. He is credited with finding the position of the constellation Ursa Major and the timing of the solstices and equinoxes. As an engineer, he helped the Lydian army cross the Halys River. Plutarch wrote that, at that time, Thales was the only one who had turned philosophy from mere speculation into practice.

The main source for Thales's life is Diogenes Laërtius, who wrote *Lives and Opinions of the Eminent Philosophers* in the 200s AD—about 800 years after Thales died. Diogenes relied on sources that often contained unreliable or made-up information. What is certain is that Thales came from Miletus, a trading city at the mouth of the Maeander River, near modern Didim, Turkey. His dates are roughly known from a few datable events. The historian Herodotus, writing in the 400s BC, says Thales predicted a solar eclipse in 585 BC. Assuming a person's peak (or floruit) happens at age 40, the 100s-BC chronicler Apollodorus of Athens placed Thales's birth around 625 BC.

Thales's ancestry is unclear. Herodotus called him "a Phoenician by remote descent." Diogenes Laërtius quotes Herodotus, Duris, and Democritus, who all agree that Thales was the son of Examyas and Cleobulina, belonged to the Thelidae (Phoenicians descended from Cadmus and Agenor), and was made a citizen of Miletus along with Neleus. But Friedrich Nietzsche and others interpret this to mean only that his ancestors were seafaring Cadmeians from Boeotia. He might have had mixed ancestry, since his father had a Carian name and his mother a Greek one. Diogenes also mentions another account: "Most writers, however, represent him as a genuine Milesian and of a distinguished family." The 1952 *Encyclopedia Britannica* concluded that Thales was most likely a native Milesian of noble birth and certainly a Greek.

Diogenes gives conflicting reports about Thales's family life. One says he married and had a son named Cybisthus (or Cybisthon), or else adopted a nephew with the same name. Another says he never married, telling his mother as a young man that it was too early, and as an older man that it was too late. Plutarch told a version where Solon visited Thales and asked why he stayed single; Thales replied that he did not want to worry about children. But years later, wanting family, he adopted his nephew Cybisthus.

Thales may have traveled to Egypt or Babylonia for trade, but there is no strong evidence he ever visited the Near East, and scholars disagree. Later writers often claimed philosophers like Thales, Pythagoras, or Eudoxus had traveled to such places to explain where their mathematical knowledge came from. Several ancient authors assume Thales visited Egypt and learned geometry there. It is possible, since Miletus had a permanent colony in Egypt called Naucratis. Some say he had close contacts with the priests of Thebes, who taught him, or that he taught them geometry. He might also have known about Egypt from others without going there. Aside from Egypt, Babylonia was the other mathematically advanced ancient civilization before the Greeks, and it was another common place to send a mathematically minded philosopher. Historians Roger L. Cooke and B.L. Van der Waerden think Babylonian mathematics influenced the Greeks, pointing to things like the base-60 system. But Cooke notes this is controversial. Other historians, like D. R. Dicks, reject the idea of Babylonian influence on Greek mathematics, since the sexagesimal system was unknown in Greece until around the time of Hipparchus (c. 190–120 BC). Herodotus wrote that the Greeks learned the gnomon from the Babylonians. Thales's follower Anaximander is credited with introducing the gnomon to the Greeks.