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This article discusses the pre-Socratic philosopher, Thales of Miletus.

See also: Thales (disambiguation)

Philosophically, the ancient Greek citizen of Miletus, Thales (ca. 640 BCE - ca. 546 BCE), was a materialist, and a monist. Aristotle (Metaphysics) says that he (Thales) was the first to consider the basic principles and originating substance of nature. When contemplating the myriad forms and changes seen in the world, the Ionian philosophers,[1] and first among them, Thales, posited a basic unity and permanence behind the world of phenomena and its changes, something which underlay the changes around them and which remained unchanged through its various manifestations.

Thales attributed all objects to a single substance, water,[2] manifesting in different ways to produce different objects. His followers also viewed the underlying reality as a single substance: Anaximenes, air; Heraclitus, fire; Anaximander, something called the Infinite, which could transmogrify into earth, air, fire, and water. Empedocles posited the latter four as separate elementary substances whose combinations made up all of the stuff of the universe. Science today starts with a many more substances, the differing atoms of the nearly 100 chemical elements. But the differing atoms all have a common set of constituents, and those may manifest as invisible strings of energy, manifesting differently to constitute different objects.

This intrepid idea of the existence of a primeval element, this idea that qualitatively matter is unitary constitutes the principal merit of Thales who in this way started twenty-five centuries ago that investigation of nature which later became one of the main fields of study of the Greek philosophers.
—Genestra Giovene Amaldi[3]

Thales could take credit for having jump-started science. Science might one day confirm Thales' view that a single 'substance', or physical process, underlie all phenomena; some 21st century scientists think they're approaching that conclusion with superstring theory.

Hawking and Mlodinow (2010) interpret Aristotle as saying that Thales was the first to develop the idea that the understanding the world was possible without resort to supernatural deities and their supernatural powers. Thales thus sought the causes of phenomena in terms of naturalistic explanations rather than the personal agency of supernatural beings. He proposed the concept of a natural law that regulated the universe, in contrast with the mythological universe governed by actions of the Olympian deities. In seeking unity in nature, he wondered about the basic principle of matter in its various forms, Thales posited a material entity, water, as this unitary substance. This is what makes him a materialist.

More important than his postulate of water as the unitary substance was his concept that there was such a substance. As Frederick Copleston stated it, summing up Thales' main philosophical contribution in his monumental History of Philosophy:

". . . the importance of this early thinker lies in the fact that he raised the question, what is the ultimate nature of the world; and not in the answer that he actually gave to that question or in his reasons, be they what they may, for giving that answer."

Thales the scientist

Later Greek writers credited Thales with numerous discoveries and advances in astronomy and geometry.

In the field of astronomy, Thales is credited by Herodotus with having predicted the year in which a solar eclipse, later identified as one which occurred in the year 585 BC. There is currently some doubt whether Thales discovered a method of prediction of solar eclipses, as Thales conceivably learned of the methods of the Babylonians for predicting solar eclipses (McKirahan 1994). It seems certain that he knew the causes of solar (and lunar) eclipses. Nevertheless, the occurrence of the eclipse, turning night into day, during a battle between the Lydians and the Medes, in the sixth year of their war, gave them pause to consider a peaceful solution (Williams 2010).

Thales is also credited with having determined the length of the solar year and the dates of the solstices, as well as the diameters of the Sun and Moon. In each case, detailed observations over lengthy periods of time are necessary as well as the conception of how to carry out the tasks.

Thales introduced mathematics to the Greek world (Anglin and Lambeck 1995). He had traveled to Egypt where he observed the practical usage of geometrical measuring techniques, whereupon he took the first steps in placing geometry on a theoretical basis. His name is associated with several basic propositions of geometry: 1) that a circle is bisected by a diameter; 2) the angles at the base of an isosceles triangle are equal; 3) the angle of a semi-circle is a right angle; 4) if two straight lines intersect each other, the opposite angles are equal, and; 5) one side and one acute angle of a right-sided triangle determine the other two sides. Using the latter, he could determine the distance of a ship from the shore.

According to Anglin and Lambeck (1995), the Egyptians already knew those propositions, but Thales was the first to 'prove' them. Thales' method of 'proof' was based on repeated, empirical measurements and induction.


  1. Note: Those progenitors of philosophy coupled philosophy as we recognize it today, with an early science (so-called natural philosophy), the latter based mostly on observations and speculative interpretations of them. Gradually, over the centuries, their philosophy diverged into two domains, philosophy proper and science proper, as we recognize them today. Yet connections remain, as 'philosophy of science' and 'scientist philosophers', as well as a 'science of philosophy'.
  2. Note: Miletus was a maritime province.
  3. Amaldi GG. (1966) The Nature of Matter: Physical Theory from Thales to Fermi. Chicago: University of Chicago Press, 1966. ISBN 0226016617 (1982 reprint).