Around 585 BCE, in the middle of a battle between the Lydians and the Medes in what is now Turkey, the sky went dark. A total solar eclipse turned day into night. Both armies, terrified by what they interpreted as a divine warning, immediately ceased fighting and negotiated peace.
According to the Greek historian Herodotus, one man had predicted this eclipse in advance: Thales of Miletus. Whether the prediction was real or legendary is debated by historians, but the story captures something essential about Thales. He was the first person in the Western tradition who attempted to explain natural phenomena not through mythology or divine intervention but through observation, reasoning, and natural causes. He was, by most accounts, the first scientist in history.
The Man from Miletus
Thales was born around 624 BCE in Miletus, a prosperous Greek city on the coast of Ionia (modern-day western Turkey). Miletus was one of the wealthiest and most cosmopolitan cities in the ancient world, a trading hub with connections to Egypt, Babylon, and Phoenicia. This location mattered: Thales almost certainly encountered Babylonian astronomical records and Egyptian mathematical techniques through trade, and these foreign traditions influenced his thinking.
Almost nothing about Thales’s life is known with certainty. He left no written works that survive. Everything we know about him comes from later writers, principally Aristotle (who lived two centuries after Thales) and Diogenes Laërtius (who lived six centuries later). The stories about Thales are a mixture of genuine historical memory and later embellishment, and separating fact from legend is often impossible.
What is clear is that the ancient Greeks themselves regarded Thales as the founder of their philosophical and scientific tradition. He was consistently named first among the Seven Sages of Greece. Aristotle identified him as the first natural philosopher. Every subsequent Greek thinker who studied nature, from Pythagoras to Euclid to Archimedes, worked in a tradition that Thales initiated.
Water as the First Principle
Thales’s most famous philosophical claim is that water is the fundamental substance of the universe. Everything, he argued, is ultimately made of water or derived from it. The earth floats on water. Life depends on water. Water can become solid (ice), liquid, or gas (steam), suggesting that all transformations in nature are transformations of this single substance.
By modern standards, this is wrong. But the importance of Thales’s claim lies not in its content but in its method. He was proposing that the bewildering variety of natural phenomena could be explained by a single underlying principle, without invoking gods, spirits, or supernatural forces. This was a radical departure from the mythological explanations that preceded him.
The Babylonians explained the world through the actions of Marduk and Tiamat. The Egyptians attributed natural events to the will of Ra and Osiris. The Greeks themselves had an elaborate mythology in which every aspect of nature was governed by a specific deity. Thales said: forget the gods. Look at nature itself. Find the principle that explains what you observe.
This shift from mythological to naturalistic explanation is the founding act of Western science. Every scientist who has ever proposed a natural law, from Newton’s gravitation to Darwin’s natural selection to Einstein’s relativity, is working within the intellectual tradition that Thales established.
Thales the Mathematician
Thales is also credited with several fundamental results in geometry, though the attributions are uncertain. According to later Greek writers, he proved or demonstrated the following:
- A circle is bisected by its diameter
- The base angles of an isosceles triangle are equal
- Vertical angles formed by two intersecting lines are equal
- Two triangles are congruent if they share two angles and a side (the ASA criterion)
- An angle inscribed in a semicircle is a right angle (Thales’s theorem)
Whether Thales actually proved these results in the rigorous sense that later Greek mathematicians would demand is unclear. He may have demonstrated them through practical examples rather than formal proofs. But the tradition credits him with introducing the idea that geometric statements must be demonstrated, not merely asserted. This is the seed from which the entire tradition of mathematical proof grew, reaching its full expression in Euclid’s Elements two and a half centuries later.
Thales is also said to have used geometric reasoning for practical purposes. One famous story describes him measuring the height of the Egyptian pyramids by comparing the length of their shadows with the shadow of a stick of known height. If the story is true, it demonstrates an early application of similar triangles, a concept that would become one of the central tools of geometry.
The Eclipse Prediction
The eclipse prediction attributed to Thales has generated centuries of scholarly debate. A total solar eclipse did occur on May 28, 585 BCE, visible from Asia Minor, and Herodotus’s account of the battle is generally considered reliable. But could Thales actually have predicted it?
The Babylonians had developed sophisticated methods for predicting lunar eclipses based on the Saros cycle, a period of approximately 18 years after which eclipses tend to recur. It is possible that Thales learned of these methods through trade contacts and applied them to predict a solar eclipse. However, predicting exactly where a solar eclipse will be visible requires calculations far beyond anything available in the 6th century BCE. Most historians believe that Thales may have predicted that an eclipse was likely within a certain period (perhaps a year) rather than pinpointing the exact date and location.
Even this more modest achievement would have been remarkable. It would mean that Thales understood that eclipses are natural, predictable events rather than divine signs, and that he had access to enough astronomical data to make a rough prediction. The story, whether literally true or not, reflects the ancient conviction that Thales represented a new kind of thinking: one that sought natural explanations and used observation and reasoning to anticipate events.
The Milesian School
Thales founded what historians call the Milesian school of natural philosophy. His two most important successors, both also from Miletus, extended his project of seeking natural explanations for the world.
Anaximander (c. 610 to 546 BCE), Thales’s student, rejected water as the fundamental substance and proposed instead the apeiron (the infinite or indefinite), an unlimited, formless substance from which all things emerge and to which they return. Anaximander also proposed that the Earth hangs freely in space (not supported by water, as Thales claimed) and that humans evolved from fish-like creatures, an astonishingly prescient intuition.
Anaximenes (c. 586 to 526 BCE), a student of Anaximander, proposed air as the fundamental substance. He argued that air becomes fire when rarefied (thinned) and becomes water and earth when condensed (compressed). This mechanism of rarefaction and condensation was an early attempt to explain qualitative changes through quantitative processes.
Together, these three Milesian thinkers established the program that would define Greek natural philosophy for the next three centuries: seeking a fundamental substance or principle that explains the diversity of nature, using observation and reason rather than mythology, and proposing specific, testable (at least in principle) claims about the physical world.
From Thales to Euclid
The mathematical tradition that Thales initiated reached its definitive form in Euclid’s Elements, written around 300 BCE. Euclid did not merely collect geometric results. He organized them into a deductive system, starting from a small set of definitions, postulates, and axioms and deriving every subsequent theorem through rigorous logical proof.
The distance from Thales’s practical demonstrations to Euclid’s formal proofs is enormous, but the line connecting them is direct. Thales insisted that geometric truths must be demonstrated. Pythagoras and his followers developed the idea further. Hippocrates of Chios (not the physician) wrote the first known Elements. And Euclid brought the entire tradition to completion in a work that remained the standard textbook of geometry for over two thousand years.
For those interested in the visual tradition of scientific achievement that runs from Thales through the centuries, Kronecker Wallis’s Portraying Science collection presents portraits of the scientists and mathematicians who built on the foundations that Thales laid.
The Beginning of Everything
Thales of Miletus stands at the beginning of Western science, philosophy, and mathematics. He asked the first scientific question (what is the world made of?), proposed the first naturalistic answer (water), introduced the first geometric demonstrations, and predicted the first astronomical event using reasoning rather than divination. None of these achievements may be literally as tradition describes them. But the tradition itself is significant: the ancient world recognized Thales as the person who started something genuinely new.
What Thales started was the conviction that the universe is comprehensible. That natural events have natural causes. That those causes can be discovered through observation and reasoning. That knowledge is not the exclusive property of priests and oracles but is available to anyone willing to look carefully and think clearly. Twenty-six centuries later, this conviction remains the foundation of every scientific investigation on Earth. It began with a man from Miletus who looked at the world and saw not the work of gods but the workings of nature.
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