Thales of Miletus (pronounced THAY-leez; Ancient Greek: Θαλῆς; around 626 to 545 BC) was an early Greek philosopher from Miletus in Ionia, which is in modern-day Turkey. He was one of the Seven Sages, important leaders in ancient Greece.

Starting in the 18th century, many historians began to see Thales as the first philosopher in the Greek tradition. He moved away from using myths to explain the world and instead used natural explanations and logical thinking. Because of this, he is often called the first person to study mathematics, science, and deductive reasoning.

Thales believed that all things in nature come from a single basic substance, which he thought was water. He also believed the Earth floated on water. His ideas influenced many other philosophers of his time.

In mathematics, Thales is known for Thales’s theorem, and the intercept theorem is sometimes called Thales’s theorem too. He is said to have measured the heights of pyramids and the distance of ships from the shore. In science, Thales was an astronomer who predicted weather changes and a solar eclipse. He is also credited with discovering the position of the constellation Ursa Major and the times of the solstices and equinoxes. As an engineer, he helped the Lydian army cross the Halys River. Plutarch wrote that “at that time, Thales alone had raised philosophy from mere speculation to practice.”

Life

The main source of information about Thales's life and work is a writer named Diogenes Laërtius, who lived in the third century AD. He wrote a book called Lives and Opinions of the Eminent Philosophers. Although this is the only detailed record of Thales's life, Diogenes wrote about 800 years after Thales died, and some of his sources may have included information that was not accurate. We know Thales was born in Miletus, a city known for trade, located at the mouth of the Maeander River, near modern-day Didim, Turkey.

The exact dates of Thales's life are not certain, but historians use some events from ancient sources to estimate them. According to the historian Herodotus, who wrote in the 5th century BC, Thales predicted a solar eclipse in 585 BC. Another writer, Apollodorus of Athens, who lived in the 2nd century BC, estimated that Thales was born around 625 BC, assuming he reached his peak at age 40.

While it is likely that Thales was Greek, like most people from Miletus, Herodotus described him as "a Phoenician by remote descent." Diogenes Laërtius mentions that Herodotus, along with other writers, claimed Thales was the son of Examyas and Cleobulina, and part of a noble family called the Thelidae, who were originally from Phoenicia. These people had been exiled from their homeland and later became citizens of Miletus. However, some scholars, like Friedrich Nietzsche, suggest that this might mean Thales's ancestors were seafarers from Boeotia. It is also possible Thales had mixed heritage, as his father had a Carian name and his mother had a Greek name. Diogenes Laërtius also notes that some writers believed Thales was a true native of Miletus from a respected family. Encyclopedia Britannica (1952) concluded that Thales was most likely a Greek from Miletus of noble birth.

Diogenes Laërtius also records conflicting stories about Thales's marriage. One account says he married and either had a son named Cybisthus or adopted his nephew of the same name. Another claims he never married, telling his mother as a young man that it was too early to marry and as an older man that it was too late. Plutarch, an ancient writer, wrote that when Solon visited Thales, he asked why he remained single. Thales replied that he disliked the responsibility of raising children. Later, he adopted his nephew Cybisthus.

The culture of Archaic Greece was influenced by civilizations in the Levant and Mesopotamia. Some sources say Thales was involved in trade and may have traveled to Egypt or Babylonia. However, there is no strong evidence that he visited these regions, and scholars debate this. Many ancient writers attributed travel to the Near East to philosophers, including Thales, to explain how they gained mathematical knowledge.

Some ancient authors believed Thales visited Egypt, where he learned geometry. This is possible because Miletus had a colony in Egypt called Naucratis. It is also said that Thales had close contact with Egyptian priests, who taught him, or that he taught them geometry. However, he might have learned about Egypt through others without visiting.

Babylonia, another ancient civilization with advanced mathematics, was also commonly linked to philosophers who traveled there. Historians like Roger L. Cooke and B.L. Van der Waerden believe Babylonian mathematics influenced the Greeks, pointing to the use of the base-60 number system. However, this view is controversial. Other historians, such as D.R. Dicks, argue that Babylonian influence on Greek math was unlikely before the time of Hipparchus (around 190–120 BC), when the base-60 system was not widely known.

Herodotus wrote that the Greeks learned the gnomon from the Babylonians. Thales's student, Anaximander, is credited with introducing the gnomon to Greece. Herodotus also claimed the Greeks adopted the practice of dividing the day into 12 parts and the concept of the polos from Babylonians. However, some historians, like L. Zhmud, argue that the gnomon was known to both Egyptians and Babylonians, the division of the day into 12 parts was already used by Egyptians as early as the 2nd millennium BC, and the polos was not used outside Greece at that time.

Thales is one of the Seven Sages of Greece, a group of wise leaders and founders of ancient Greece. While the specific seven may vary, four are widely accepted: Thales, Solon of Athens, Pittacus of Mytilene, and Bias of Priene. Diogenes Laërtius wrote that the Seven Sages were chosen during the time when Damasius was in charge of Athens, around 582 BC, and that Thales was the first sage.

The Seven Sages were associated with the Delphic maxims, sayings attributed to each sage and inscribed on the Temple of Apollo at Delphi. Thales is linked to the most famous maxim: "Know thyself" (gnothi seauton). According to the 10th-century Byzantine encyclopedia Suda, this phrase was used to warn people not to boast about their abilities and to ignore the opinions of others.

Diogenes Laërtius tells stories about a gold tripod or bowl awarded to the wisest person. In one version, a man named Bathycles of Arcadia left instructions in his will for the bowl to be given to the wisest person. It was eventually given to Thales, passed among the sages, and returned to him. Thales then sent it to Apollo at Didyma, dedicating it to the god.

Diogenes Laërtius also notes that Thales gained fame for advising the people of Miletus not to join a military alliance with the Lydians. This decision may have helped Miletus avoid conflict. Later, when the Lydian king Croesus was defeated by the Persian king Cyrus the Great, Cyrus spared Miletus because it had not taken part in the war. Cyrus admired Croesus's wisdom and the influence of the sages, and he followed their advice. He also suggested that the Ionian cities should be treated as districts (demos) under Persian rule.

Miletus received favorable terms from Cyrus, while other Ionian cities formed a league of twelve and were later conquered by the Persians.

Theories and studies

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Early Greeks, and other civilizations before them, often explained natural events by referring to the will of human-like gods and heroes. Instead, Thales tried to explain these events using logical ideas that looked at natural processes themselves— Logos rather than mythos.

Aristotle believed that Thales was the first natural philosopher (physiologoi), but this idea was not widely accepted by others, even those in Aristotle's Peripatetic school, until the eighteenth century. Also, while the other Seven Sages were only law-givers and statesmen, not philosophers, Plutarch noted that "it would seem that Thales was the only wise man of the time who carried his speculations beyond the realm of the practical."

Thales's most famous idea was his philosophical and cosmological belief that all things are made of water. This idea comes to us through a passage from Aristotle's Metaphysics. In the work, Aristotle reported Thales's theory that the arche, or the starting point of nature, was a single material substance: water. Aristotle then gave his own thoughts about why Thales may have believed this (though Aristotle did not believe it himself).

While Aristotle's thoughts on why Thales believed water was the starting point of matter are his own, his statement that Thales believed this is generally accepted as coming from Thales. Writing centuries later, Diogenes Laërtius also said that Thales taught "Water constituted (ὑπεστήσατο, 'stood under') the principle of all things."

According to Aristotle:

Aristotle further adds:

The 1870 book Dictionary of Greek and Roman Biography and Mythology noted:

Most agree that Thales's influence on thought was the idea of the unity of substance. Not only the idea that all is water, but the deeper idea that all is one. For example, Friedrich Nietzsche, in his Philosophy in the Tragic Age of the Greeks, wrote:

Megiston topos: apanta gar chorei (Μέγιστον τόπος· ἄπαντα γὰρ χωρεῖ.)

The greatest is space, for it holds all things.

Thales was known for introducing the use of geometry in theory and practice to Greece, and he is considered the first person in the Western world to apply deductive reasoning to geometry, making him the West's "first mathematician." He is also credited with the West's oldest definition of number: a "collection of units," following the Egyptian view.

The evidence for Thales's importance comes from a book by Proclus, who lived a thousand years later but is believed to have had a copy of Eudemus's lost book, History of Geometry (4th century BC). Proclus wrote that Thales was the first to visit Egypt and bring the Egyptian study of mathematics to Greece, and that Thales "himself discovered many propositions and disclosed the underlying principles of many others to his successors, in some cases his method being more general, in others more empirical." In addition to Proclus, Hieronymus of Rhodes (3rd century BC) also cites Thales as the first Greek mathematician.

Proclus attributes to Thales the discovery that a circle is bisected by its diameter, that the base angles of an isosceles triangle are equal, and that vertical angles are equal. Two fundamental theorems of elementary geometry are usually called Thales's theorem: one of them relates to a triangle inscribed in a circle with the circle's diameter as one side; the other, also called the intercept theorem, is about an angle intercepted by two parallel lines, forming a pair of similar triangles.

Modern scholars are skeptical that anyone in Thales's time was producing mathematical proofs to the standard of later Greek mathematics, though not enough direct evidence remains to draw firm conclusions. While Thales may have discovered some basic geometric relations and provided some justification for them, attribution to him of formal proofs is now thought to represent speculative rationalization and reconstruction by later authors, rather than concrete accomplishments of Thales himself or his contemporaries.

According to one author, while visiting Egypt, Thales observed that when the Egyptians drew two intersecting lines, they would measure the vertical angles to make sure they were equal. Thales concluded that one could prove that all vertical angles are equal if one accepted some general notions such as: all straight angles are equal, equals added to equals are equal, and equals subtracted from equals are equal.

Pamphila says that, having learned geometry from the Egyptians, Thales was the first to inscribe in a circle a right-angled triangle, whereupon he sacrificed an ox. This is sometimes cited as history's first mathematical discovery. Due to the variations among testimonies, such as the story of the ox sacrifice being credited to Pythagoras upon the discovery of the Pythagorean theorem rather than Thales, some historians (such as D. R. Dicks) question whether such anecdotes have any historical worth whatsoever.

It is believed the Babylonians knew the theorem for special cases. The theorem is mentioned and proved as part of the 31st proposition in the third book of Euclid's Elements. Dante's Paradiso refers to Thales's theorem in the course of a speech.

The story is told in Diogenes Laërtius, Pliny the Elder, and Plutarch, sourced from Hieronymus of Rhodes, that when Thales visited Egypt, he measured the height of the pyramids by their shadows at the moment when his own shadow was equal to his height. According to Plutarch, it pleased the pharaoh Amasis. More practically, Thales was said to have the ability to measure the distances of ships at sea.

These stories indicate familiarity with the intercept theorem, and for this reason, the 26th proposition in the first book of Euclid's Elements was attributed to Thales. They also indicate that he was familiar with the Egyptian seked, or seqed, the ratio of the run to the rise of a slope (cotangent). According to Kirk & Raven, all you need for this feat is three straight sticks pinned at one end and knowledge of your altitude. One stick goes vertically into the ground. A second is made level. With the third, you sight the ship and calculate the seked from the height of the stick and its distance from the point of insertion to the line of sight.

Thales was also a noted astronomer, acknowledged in antiquity for describing the position of Ursa Minor, and he thought the constellation might be useful as a guide for navigation at sea. He calculated the duration of the year and the timings of the equinoxes and solstices. He is also attributed with calculating the position of the Pleiades.

Plutarch indicates that in his day (c. AD 100) there was an extant work, the Astronomy, composed in verse and attributed to Thales. While some say he left no writings, others say that he wrote On the Solstice and On the Equinox. The Nautical Star-guide has also been attributed to him, but this was disputed even in ancient times. No writing attributed to him has survived. Lobon of Argus asserted that the writings of Thales amounted to two hundred lines.

Thales thought the Earth must be a flat disk or mound of land and dirt floating in an expanse of water. Heraclitus Homericus states that Thales drew his conclusion from seeing moist substance turn into air, slime, and earth. It seems likely that Thales viewed the land as coming from the water on which it floated and the oceans that surround it, perhaps inspired by observing silt deposits.

He thought the stars were balls of earth on fire. He seemed to correctly gather that the moon reflects the Sun's light. A crater on the Moon is named in his honor.

Rather than assuming that earthquakes were the result of supernatural whims, Thales explained them through natural causes.

Death and legacy

Diogenes Laërtius says that Apollodorus wrote that Thales died at the age of 78 during the 58th Olympiad (548–545 BC). His death was caused by heat stroke and thirst while watching the games.

Thales had a great influence on other Greek thinkers and on Western history. However, because there are few sources about Thales and the surviving sources often disagree, scholars debate how much influence he had and which later Greek philosophers and mathematicians were affected by him.

The first three philosophers in the Western tradition were all cosmologists from Miletus. Thales was the first, followed by Anaximander, who was then followed by Anaximenes. These three are called the Milesian school. According to the Suda, Thales was the "teacher and kinsman" of Anaximander. While Thales believed water was the basic substance of everything, Anaximander thought everything came from the "apeiron," or the unlimited. Anaximenes, who followed Anaximander, believed air was the basic substance, similar to Thales’ belief about water.

John Burnet (1892) noted

As one of the first Greek mathematicians, Thales is also considered to have influenced Pythagoras. According to Iamblichus, Pythagoras "benefited from Thales’ instruction in many ways, but his greatest lesson was learning the value of saving time." Early sources say that Pythagoras, who was a student of Anaximander, visited Thales as a young man. Thales advised him to travel to Egypt to continue his studies in philosophy and mathematics.

Thales was also considered the teacher of the astronomer Mandrolytus of Priene. It is possible he was also the teacher of Cleostratus of Tenedos.