Thales of Miletus (ca. 624 BC–ca. 546 BC), was a Greek philosopher and mathematician and one of the seven wise men of antiquity. He was the first Greek philosopher to take an interest in mathematics and science. He was the first to apply logical proof to geometry, and proved the six basic propositions of geometry. When he traveled to Egypt he calculated the height of pyramids using geometry, and would also calculate the distance of ships from the shore. Thales was born and lived in Miletus, Ionia, now western Turkey. According to Herodotus, he was of Phoenician descent.
Thales the philosopher
Philosophically, Thales was a materialist, and a monist. Aristotle 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, 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 the various manifestations.
Thales sought the causes of phenomena in terms of naturalistic explanations rather than the personal agency of supernatural beings and thus proposed the concept of a law regulated universe, in contrast with the mythological universe of the Olympian deities. In seeking for the basic principle of matter, Thales posited a material entity, water, as this basic substance. This is what makes him a materialist, but it was his concept that there was such a basic substance which is even more important.
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
In the field of astronomy, Thales is credited by Herodotus with having predicted a solar eclipse, later identified as one which occurred in the year 585 BC. Although, in the absence of Thales' actual writings, there is currently some doubt as to the authenticity and nature of this prediction, it seems certain that he knew the causes of solar (and lunar) eclipses. He 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, who had travelled to Egypt where he observed the practical usage of geometrical measuring techniques, took the first steps in placing geometry on a theoretical basis. He is credited with having demonstrated several basic propositions of geometry. Thales' method of "proof" was based on repeated, empirical measurements and induction.
Among the propositions which he demonstrated in this manner are:
- that a circle is bisected by a diameter
- the angles at the base of an isoceles triangle are equal
- the angle of a semi-circle is a right angle
- if two straight lines intersect each other, the opposite angles are equal, and
- he demonstrated certain properties of similar triangles, including the principle of ratio, or proportionality. It is this latter property (of simiilar triangles) which Thales used to measure the previously unknown height of the pyramids as well as the distance of ships at sea.
There are no surviving writings of Thales. Our knowledge of Thales' life and thought comes to us in the form of commentaries by subsequent writers, principally Aristotle (with particular reference to his Metaphysics), who commented on all of his predecessors, including Thales. Some modern discussions are listed below.
- Internet Encyclopedia of Philosophy entry on Thales - contains detailed references to many of the ancient writings discussing Thales
- Thales of Miletus
- Barnes, J., Early Greek Philosophy (London, 1987)
- Copleston, F.C., History of Philosophy, Vol 1: Greece and Rome (Part 1 is a section on Pre-Socratic Philosophers)
- Kirk, G.S., Raven, J.E., and Schofield, M., The Presocratic Philosophers (Cambridge, 1990)