The Greek natural philosopher Thales (ca. 624-ca. 545 B.C.) founded the Ionian school of ancient Greek thinkers.

Thales was descended, according to the historian Herodotus, from Phoenicians who had settled in Miletus, a thriving Greek seaport on the west coast of Asia Minor (now Turkey). His mother, however, bore a Greek name. Thales's interest in the heavens was so well known that the philosopher Plato picked him as the example of the impractical student: while gazing upward and scanning the stars, he fell into a well.

Thales became so famous for his practical shrewdness and theoretical wisdom that in later times he began to be honored for having made important discoveries whose true origins were not known then and in some cases are still obscure. The most spectacular of these supposed achievements was his alleged prediction of a total solar eclipse (presumably that of May 28, 585 B.C.), at a time when the information needed to foresee such an event was not yet possessed by anybody. Indeed it is significant that, according to Herodotus, the time mentioned in Thales's prediction was limited only to "the year in which the eclipse occurred." Month and day were not specified, nor was there any indication of the portion of the earth's surface from which the eclipse would be visible.

Thales was also falsely credited with having found out that an eclipse of the sun is caused by the interposition of the opaque moon between the sun and the earth. However, the real nature of the moon as a dark, non-self-luminous body was first disclosed about a century after the death of Thales. In like manner he was praised for having determined the sun's apparent diameter, yet this approximately correct value was first ascertained, according to the mathematician Archimedes, some 300 years after Thales by an accomplished astronomer.

The first proof that a circle is bisected by its diameter was ascribed in antiquity to Thales. But in his lifetime the Greeks had not yet begun to enunciate geometrical theorems and to demonstrate them step by step. Hence, the ancient attribution to Thales of the earliest proof of the equality of the vertical angles formed by the intersection of two straight lines is now discarded as a misplaced anticipation of a later stage in the development of Greek geometry.

In assigning to Thales the belief that "everything is full of gods, " Aristotle suggested that the Milesian perhaps derived this opinion from those who held that soul pervades the entire universe. With regard to Thales's conception of soul, Aristotle remarked that "on the basis of what people remember, Thales apparently assumed that soul causes motion, if he really said that the magnet has a soul since it attracts iron." Evidently Aristotle did not have in his hands the writings later ascribed to Thales. It is, in fact, entirely doubtful that Thales set his ideas down in written form.

In Aristotle's time the oldest traditional explanation of what held the earth up was that it rested on water. "They say that Thales the Milesian espoused this view, " Aristotle states, "because the earth remains afloat like wood or some other such thing." Aristotle wryly adds: "as though the same reasoning with regard to the earth did not apply also to the water supporting the earth." Thales based his conception of water as the fundamental principle of the universe, according to Aristotle's surmise, "on the observation that the nourishment of all things was moist, and that heat itself arises from this source and is kept alive by it." As a biologist, Aristotle appended the further reason that "the seeds of all things have a moist nature, " and one of his commentators contributed the remark that "dead things dry up."

The concept of the primacy of water may have been imported by Thales from the Egyptians, "who express this idea in mythical form." Whether or not Thales was familiar with the Egyptian water myths or the similar Mesopotamian stories and the corresponding notions in the Hebrew Bible, the framework of his thought was confined to the world of nature. Even though he overestimated the importance of water, its absence from the surface of the moon in part explains the nonexistence of life on this natural satellite. After the accomplishments actually due to Thales's successors have been stripped away from his previously exaggerated reputation, through the mists of early intellectual history Thales is dimly glimpsed as having turned rational thought to the demythologized understanding of the physical universe.

Further Reading on Thales

Modern discussions of Thales are necessarily limited by the fact that nothing of his has survived, and what may be gleaned from the writings of others is too little to permit the reconstruction of his thought. Nevertheless, G. S. Kirk and J. E. Raven present what is known about him in The Presocratic Philosophers (1964), as do John Burnet in Early Greek Philosophy (4th ed. 1930) and Kathleen Freeman in The Presocratic Philosophers (1953). Thales and his position in the development of Greek thought are also discussed in George Sarton, The Study of the History of Science (1936), and in Albin Lesky, A History of Greek Literature (1966).

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