Thales of Miletus (ca. 635 BC-543 BC), also known as Thales the Milesian, was a pre-Socratic Greek philosopher and one of the Seven Sages of Greece. Many regard him as the first philosopher in the Greek tradition as well as the father of science.

Life:

Thales (pronounced THAY-lees) was born in the Greek city of Miletus (on the Ionian coast of modern-day Turkey) in about 624 or 625 B.C. (an estimate based on his age at death). The 3rd Century A.D. historian Diogenes Laërtius reported that his parents were Examyas and Cleobulina of the noble Milesian family of Thelidae (and descended from Agenor and Cadmus of ancient Thebes, Greece), although other sources suggest that his parents may have been Phoenician (from the modern-day region of Lebanon, Israel and Syria).

Details of his life are sketchy and often contradictory. Some reports suggest that he married and had a son, Cybisthus (or Cybisthon) or possibly adopted a nephew of the same name, while other reports suggest that he never married. Some say that he left no writings; others that he wrote at least two works, "On the Solstice" and "On the Equinox" (neither have survived). Some anecdotes suggest that Thales was involved in business and politics, and at one point bought up all the olive presses in Miletus after predicting a good harvest for a particular year (either to make money or merely to demonstrate that he could use his intelligence to enrich himself if he had wanted to).

His involvement in local politics is also rather anecdotal in nature, but Thales apparently impressed both sides of the ongoing conflict between the Lydians, Medes and Persians over the fate of the region of Ionia, when he predicted an eclipse of the sun which brought fighting to a standstill. He was also reportedly involved in the negotiations which followed the hostilities, and managed to obtain favourable terms for Miletus.

Thales is said to have died of dehydration while watching a gymnastics contest in 546 or 547 B.C., at the age of 78 (although other reports have him living to the age of 90).



The writing of Thales:

Doubts have always existed about whether Thales wrote anything, but a number of ancient reports credit him with writings. Simplicius (Diels, Dox. p. 475) specifically attributed to Thales authorship of the so-called Nautical Star-guide. Diogenes Laertius raised doubts about authenticity, but wrote that ‘according to others [Thales] wrote nothing but two treatises, one On the Solstice and one On the Equinox‘ (D.L. I.23). Lobon of Argus asserted that the writings of Thales amounted to two hundred lines (D.L. I.34), and Plutarch associated Thales with opinions and accounts expressed in verse (Plutarch, De Pyth. or. 18. 402 E). Hesychius, recorded that ‘[Thales] wrote on celestial matters in epic verse, on the equinox, and much else’ (DK, 11A2). Callimachus credited Thales with the sage advice that navigators should navigate by Ursa Minor (D.L. I.23), advice which may have been in writing.

Diogenes mentions a poet, Choerilus, who declared that ‘[Thales] was the first to maintain the immortality of the soul’ (D.L. I.24), and in De Anima, Aristotle’s words ‘from what is recorded about [Thales]‘, indicate that Aristotle was working from a written source. Diogenes recorded that ‘[Thales] seems by some accounts to have been the first to study astronomy, the first to predict eclipses of the sun and to fix the solstices; so Eudemus in his History of Astronomy. It was this which gained for him the admiration of Xenophanes and Herodotus and the notice of Heraclitus and Democritus’ (D.L. I.23). Eudemus who wrote a History of Astronomy, and also on geometry and theology, must be considered as a possible source for the hypotheses of Thales. The information provided by Diogenes is the sort of material which he would have included in his History of Astronomy, and it is possible that the titles On the Solstice, and On the Equinox were available to Eudemus. Xenophanes, Herodotus, Heraclitus and Democritus were familiar with the work of Thales, and may have had a work by Thales available to them.

Proclus recorded that Thales was followed by a great wealth of geometers, most of whom remain as honoured names. They commence with Mamercus, who was a pupil of Thales, and include Hippias of Elis, Pythagoras, Anaxagoras, Eudoxus of Cnidus, Philippus of Mende, Euclid, and Eudemus, a friend of Aristotle, who wrote histories of arithmetic, of astronomy, and of geometry, and many lesser known names. It is possible that writings of Thales were available to some of these men.

Any records which Thales may have kept would have been an advantage in his own work. This is especially true of mathematics, of the dates and times determined when fixing the solstices, the positions of stars, and in financial transactions. It is difficult to believe that Thales would not have written down the information he had gathered in his travels, particularly the geometry he investigated in Egypt and his measuring of the height of the pyramid, his hypotheses about nature, and the cause of change.

Proclus acknowledged Thales as the discoverer of a number of specific theorems (A Commentary on the First Book of Euclid’s Elements 65. 8-9; 250. 16-17). This suggests that Eudemus, Proclus’s source had before him the written records of Thales’s discoveries. How did Thales ‘prove’ his theorems if not in written words and sketches? The works On the Solstice, On the Equinox, which were attributed to Thales (D.L. I.23), and the ‘Nautical Star-guide, to which Simplicius referred, may have been sources for the History of Astronomy of Eudemus (D.L. I.23).



Thales and water:

Aristotle defined wisdom as knowledge of certain principles and causes (Metaph. 982 a2-3). He commenced his investigation of the wisdom of the philosophers who preceded him, with Thales, the first philosopher, and described Thales as the founder of natural philosophy (Metaph. 983 b21-22). He recorded: ‘Thales says that it is water’. ‘it’ is the nature, the archê, the originating principle. For Thales, this nature was a single material substance, water. Despite the more advanced terminology which Aristotle and Plato had created, Aristotle recorded the doctrines of Thales in terms which were available to Thales in the sixth century BCE Aristotle made a definite statement, and presented it with confidence. It was only when Aristotle attempted to provide the reasons for the opinions that Thales held, and for the theories that he proposed, that he sometimes displayed caution.



Thales and mythology:

Those who believe that Thales inherited his views from Greek or Near-Eastern sources are wrong. Thales was esteemed in his times as an original thinker, and one who broke with tradition and not as one who conveyed existing mythologies. Aristotle unequivocally recorded Thales’s hypothesis on the nature of matter, and proffered a number of conjectures based on observation in favour of Thales’s declaration (Metaph. 983 b20-28). His report provided the testimony that Thales supplanted myth in his explanations of the behaviour of natural phenomena. Thales did not derive his thesis from either Greek or non-Greek mythological traditions.

Thales would have been familiar with Homer’s acknowledgements of divine progenitors but he never attributed organization or control of the cosmos to the gods. Aristotle recognized the similarity between Thales’s doctrine about water and the ancient legend which associates water with Oceanus and Tethys, but he reported that Thales declared water to be the nature of all things. Aristotle pointed to a similarity to traditional beliefs, not a dependency upon them. Aristotle did not call Thales a theologian in the sense in which he designated ‘the old poets’ (Metaph. 1091 b4) and others, such as Pherecydes, as ‘mixed theologians’ who did not use ‘mythical language throughout’ (Metaph. 1091 b9). To Aristotle, the theories of Thales were so obviously different from all that had gone before that they stood out from earlier explanations. Thales’s views were not ancient and primitive. They were new and exciting, and the genesis of scientific conjecture about natural phenomena. It was the view for which Aristotle acknowledged Thales as the founder of natural philosophy.



Thales primary principle:

The problem of the nature of matter, and its transformation into the myriad things of which the universe is made, engaged the natural philosophers, commencing with Thales. For his hypothesis to be credible, it was essential that he could explain how all things could come into being from water, and return ultimately to the originating material. It is inherent in Thales’s hypotheses that water had the potentiality to change to the myriad things of which the universe is made, the botanical, physiological, meteorological and geological states. In Timaeus, 49B-C, Plato had Timaeus relate a cyclic process. The passage commences with ‘that which we now call “water” ‘, and describes a theory which was possibly that of Thales. Thales would have recognized evaporation, and have been familiar with traditional views, such as the nutritive capacity of mist and ancient theories about spontaneous generation, phenomena which he may have ‘observed’, just as Aristotle believed he, himself had (Hist. An. 569 b1; Gen. An. 762 a9-763 a34), and about which Diodorus Siculus (I.7.3-5; 1.10.6), Epicurus (ap. Censorinus, D.N. IV.9), Lucretius (De Rerum Natura , V.783-808) and Ovid (Met. I.416-437) wrote.

When Aristotle reported Thales’s pronouncement that the primary principle is water, he made a precise statement: ‘Thales says that it [the nature of things] is water’ (Metaph. 983 b20), but he became tentative when he proposed reasons which might have justified Thales’s decision: ‘[Thales's] supposition may have arisen from observation . . . ‘ (Metaph. 983 b22). It was Aristotle’s opinion that Thales may have observed, ‘that the nurture of all creatures is moist, and that warmth itself is generated from moisture and lives by it; and that from which all things come to be is their first principle’ (Metaph. 983 b23-25). Then, in the lines 983 b26-27, Aristotle’s tone changed towards greater confidence. He declared: ‘Besides this, another reason for the supposition would be that the semina of all things have a moist nature . . . ‘ (Metaph. 983 b26-27). In continuing the criticism of Thales, Aristotle wrote: ‘That from which all things come to be is their first principle’ (Metaph. 983 b25).

Simple metallurgy had been practised long before Thales presented his hypotheses, so Thales knew that heat could return metals to a liquid state. Water exhibits sensible changes more obviously than any of the other so-called elements, and can readily be observed in the three states of liquid, vapour and ice. The understanding that water could generate into earth is basic to Thales’s watery thesis. At Miletus it could readily be observed that water had the capacity to thicken into earth. Miletus stood on the Gulf of Lade through which the Maeander river emptied its waters. Within living memory, older Milesians had witnessed the island of Lade increasing in size within the Gulf, and the river banks encroaching into the river to such an extent that at Priene, across the gulf from Miletus the warehouses had to be rebuilt closer to the water’s edge. The ruins of the once prosperous city-port of Miletus are now ten kilometres distant from the coast and the Island of Lade now forms part of a rich agricultural plain. There would have been opportunity to observe other areas where earth generated from water, for example, the deltas of the Halys, the Ister, about which Hesiod wrote (Theogony, 341), now called the Danube, the Tigris-Euphrates, and almost certainly the Nile. This coming-into-being of land would have provided substantiation of Thales’s doctrine. To Thales water held the potentialities for the nourishment and generation of the entire cosmos. Aëtius attributed to Thales the concept that ‘even the very fire of the sun and the stars, and indeed the cosmos itself is nourished by evaporation of the waters’ (Aëtius, Placita, I.3).



The eclipse of Thales:

Thales is acclaimed for having predicted an eclipse of the sun which occurred on 28 May 585 BCE The earliest extant account of the eclipse is from Herodotus: ‘On one occasion [the Medes and the Lydians] had an unexpected battle in the dark, an event which occurred after five years of indecisive warfare: the two armies had already engaged and the fight was in progress, when day was suddenly turned into night. This change from daylight to darkness had been foretold to the Ionians by Thales of Miletus, who fixed the date for it within the limits of the year in which it did, in fact, take place’ (Hdt. I.74). The vital points are: Thales foretold a solar eclipse; it did occur within the period he specified. How Thales foretold the eclipse is not known but there is strong opinion that he was able to perform this remarkable feat through knowledge of a cycle known as the Saros, with some attributing his success to use of the Exeligmos cycle. It is not known how Thales was able to predict the Eclipse, if indeed he did, but he could not have predicted the Eclipse by using the Saros or the Exeligmos cycles.

In addition to Herodotus, the successful prediction of the eclipse was accepted by Eudemus in his History of Astronomy and acknowledged by a number of other writers of ancient times (Cicero, Pliny, Dercyllides, Clement, Eusebius). This is how Diogenes Laertius recorded the event: ‘[Thales] seems by some accounts to have been the first to study astronomy, the first to predict eclipses of the sun, and to fix the solstices; so Eudemus in his History of Astronomy. It was this which gained for him the admiration of Xenophanes and Herodotus and the notice of Heraclitus and Democritus’ (D.L. I.23). Diogenes asserted that Herodotus knew of Thales’s work, and in naming Xenophanes, Heraclitus, and Democritus, he nominated three of the great pre-Socratics, eminent philosophers who were familiar with the work of Thales.

Modern astronomy confirms that the eclipse did occur, and was total. According to Herodotus’s report, the umbra of the eclipse of Thales must have passed over the battle field. The “un-naturalness” of a solar eclipse is eerie and chilling. All becomes hushed and there is a strong uncanny sensation of impending disaster, of being within the control of some awful power. In ancient times, the awesome phenomenon must have aroused great fear, anxiety and wonder. The combatants saw the eclipse as disapproval of their warfare, and as a warning. They ceased fighting and a peace agreement was reached between the two kings.

It is not known why Thales turned away from the traditional beliefs which attributed all natural events and man’s fortunes and misfortunes to the great family of Olympian gods, but Miletus was the most prosperous of the Ionian cities, and it cannot be doubted that the flourishing merchants believed that their prosperity resulted from their own initiative and endeavours. Thales’s great philosophical pronouncement that water is the basic principle shows that Thales gave no acknowledgement to the gods as instigators and controllers of phenomena. Thales’s hypotheses indicate that he envisaged phenomena as natural events with natural causes and possible of explanation. From his new perspective of observation and reasoning, Thales studied the heavens and sought explanations of heavenly phenomena.

It is widely accepted that Thales acquired information from Near-Eastern sources and gained access to the extensive records which dated from the time of Nabonassar (747 BCE) and which were later used by Ptolemy (Alm. III.7. H 254). Some commentators have suggested that Thales predicted the solar eclipse of 585 BCE through knowledge of the Saros period, a cycle of 223 lunar months (18 years, 10-11 days plus 0.321124 of a day) after which eclipses both of the sun and moon repeat themselves with very little change, or through knowledge of the Exeligmos cycle which is exactly three times the length of the Saros (Ptolemy, Alm. IV.2. H270). The ancients could not have predicted solar eclipses on the basis of those periodic cycles because eclipses of the sun do not repeat themselves with very little change. The extra 0.321124 of a day means that each recurring solar eclipse will be visible to the west, just under one-third of the circumference of the earth, being a period of time of almost 7.7 hours. This regression to the west could not have been known to the ancient astrologers, a fact which seems not to have been taken into account by the philosophers who attribute Thales’s success to application of one of those two cycles.

The following important fact should be noted. Some commentators and philosophers believe that Thales may have witnessed the solar eclipse of 18th May 603 BCE or have had heard of it. They accepted that he had predicted the solar eclipse of 28 May 585 BCE and reasoned from the astronomical fact of the Saros cycles and the fact that the two solar eclipses had been separated by the period of 18 years, 10 days, and 7.7 hours, and concluded that Thales had been able to predict a solar eclipse based upon the knowledge of that cycle. Two facts discount rebut those claims. First, recent research shows that the solar eclipse of 18th May 603 BCE would not have been visible in Egypt, nor in the Babylonian observation cities where the astronomers watched the heavens for expected and unusual heavenly events. The eclipse of 603 passed over the Persian Gulf, too far to the south for observation (Stephenson, personal communication, March 1999; and Stephenson, “Long-term Fluctuations”, 165-202). Even if the eclipse of 603 had been visible to the Near-Eastern astronomers, it is not possible to recognize a pattern from witnessing one event, or indeed, from witnessing two events. One may suggest a pattern after witnessing three events that are separated by equal periods of time, but the eclipse which preceded that of 603, and which occurred on 6th May 621, was not visible in Near-Eastern regions. Consequently, it could not have been recorded by the astrologer/priests who watched for unusual heavenly phenomena, and could not have been seen as forming a pattern.

It is quite wrong to say that eclipses repeat themselves with very little change, because each solar eclipse in a particular Saros occurs about 7.7 hours later than in the previous eclipse in the same Saros, and that is about 1/3 of the circumference of the earth’s circumference. Adding to the difficulty of recognizing a particular cycle is the fact that about forty-two periodic cycles are in progress continuously, and overlapping at any time. Every series in a periodic cycle lasts about 1,300 years and comprises 73 eclipses. Eclipses which occur in one periodic cycle are unrelated to eclipses in other periodic cycles.

The ancient letters prove that the Babylonians and Assyrians knew that lunar eclipses can occur only at full moon, and solar eclipses only at new moon, and also that eclipses occur at intervals of five or six months. However, while lunar eclipses are visible over about half the globe, solar eclipses are visible from only small areas of the earth’s surface. Recent opinion is that, as early as 650 BCE the Assyrian astronomers seem to have recognized the six months-five months period by which they could isolate eclipse possibilities (Steele, “Eclipse Prediction”, 429).

In other recent research Britton has analysed a text known as Text S, which provides considerable detail and fine analysis of lunar phenomena dating from Nabonassar in 747 BCE The text points to knowledge of the six-month five month periods. Britton believes that the Saros cycle was known before 525 BCE (Britton, “Scientific Astronomy”, 62) but, although the text identifies a particular Saros cycle, and graphically depicts the number of eclipse possibilities, the ancient commentary of Text S does not attest to an actual observation (Britton, “An Early Function”, 32).

There is no evidence that the Saros could have been used for the prediction of solar eclipses in the sixth century BCE, but it remains possible that forthcoming research, and the transliteration of more of the vast stock of ancient tablets will prove that the Babylonians and Assyrians had a greater knowledge of eclipse phenomena than is now known.

The Babylonian and Assyrian astronomers knew of the Saros period in relation to lunar eclipses, and had some success in predicting lunar eclipses but, in the sixth century BCE when Thales lived and worked, neither the Saros nor the Exeligmos cycles could be used to predict solar eclipses.

It is testified that Thales knew that the sun is eclipsed when the moon passes in front of it, the day of eclipse – called the thirtieth by some, new moon by others (The Oxyrhynchus Papyri, 3710). Aëtius (II.28) recorded: [Thales] says that eclipses of the sun take place when the moon passes across it in a direct line, since the moon is earthy in character; and it seems to the eye to be laid on the disc of the sun’.

There is a possibility that, through analysis of ancient eclipse records, Thales identified another cycle, the lunar eclipse-solar eclipse cycle of 23 1/2 months, the fact that a solar eclipse is a possibility 23 1/2 months after a lunar eclipse. However, lunar eclipses are not always followed by solar eclipses. Although the possibility is about 57% it is important to note that the total solar eclipse of 28th May, 585, occurred 23 1/2months after the total lunar eclipse of 4th July, 587. The wording of the report of the eclipse by Herodotus: ‘Thales . . . fixed the date for the eclipse within the limits of the year’ is precise, and suggests that Thales’s prediction was based upon a definite eclipse theory.



Mathematiques:

Five Euclidean theorems have been explicitly attributed to Thales, and the testimony is that Thales successfully applied two theorems to the solution of practical problems.

Thales did not formulate proofs in the formal sense. What Thales did was to put forward certain propositions which, it seems, he could have ‘proven’ by induction: he observed the similar results of his calculations: he showed by repeated experiment that his propositions and theorems were correct, and if none of his calculations resulted in contrary outcomes, he probably felt justified in accepting his results as proof. Thalean ‘proof’ was often really inductive demonstration. The process Thales used was the method of exhaustion. This seems to be the evidence from Proclus who declared that Thales ‘attacked some problems in a general way and others more empirically’.

DEFINITION I.17: A diameter of the circle is a straight line drawn through the centre and terminated in both directions by the circumference of the circle; and such a straight line also bisects the circle (Proclus, 124). >

PROPOSITION I.5: In isosceles triangles the angles at the base are equal; and if the equal straight lines are produced further, the angles under the base will be equal (Proclus, 244). It seems that Thales discovered only the first part of this theorem for Proclus reported: We are indebted to old Thales for the discovery of this and many other theorems. For he, it is said, was the first to notice and assert that in every isosceles the angles at the base are equal, though in somewhat archaic fashion he called the equal angles similar (Proclus, 250.18-251.2).

PROPOSITION I.15: ‘If two straight lines cut one another, they make the vertical angles equal to one another’ (Proclus, 298.12-13). This theorem is positively attributed to Thales. Proof of the theorem dates from the Elements of Euclid (Proclus, 299.2-5).

PROPOSITION I.26: ‘If two triangles have the two angles equal to two angles respectively, and one side equal to one side, namely, either the side adjoining the equal angles, or that subtending one of the equal angles, they will also have the remaining sides equal to the remaining sides and the remaining angle equal to the remaining angle’ (Proclus, 347.13-16). ‘Eudemus in his history of geometry attributes the theorem itself to Thales, saying that the method by which he is reported to have determined the distance of ships at sea shows that he must have used it’ (Proclus, 352.12-15). Thales applied this theorem to determine the height of a pyramid. The great pyramid was already over two thousand years old when Thales visited Gizeh, but its height was not known. Diogenes recorded that ‘Hieronymus informs us that [Thales] measured the height of the pyramids by the shadow they cast, taking the observation at the hour when our shadow is of the same length as ourselves’ (D.L. I.27). Pliny (HN, XXXVI.XVII.82) and Plutarch (Conv. sept. sap. 147) also recorded versions of the event. Thales was alerted by the similarity of the two triangles, the ‘quality of proportionality’. He introduced the concept of ratio, and recognized its application as a general principle. Thales’s accomplishment of measuring the height of the pyramid is a beautiful piece of mathematics. It is considered that the general principle in Euclid I.26 was applied to the ship at sea problem, would have general application to other distant objects or land features which posed difficulties in the calculation of their distances.

PROPOSITION III.31: ‘The angle in a semicircle is a right angle’. Diogenes Laertius (I.27) recorded: ‘Pamphila states that, having learnt geometry from the Egyptians, [Thales] was the first to inscribe a right-angled triangle in a circle, whereupon he sacrificed an ox’. Aristotle was intrigued by the fact that the angle in a semi-circle is always right. In two works, he asked the question: ‘Why is the angle in a semicircle always a right angle?’ (An. Post. 94 a27-33; Metaph. 1051 a28). Aristotle described the conditions which are necessary if the conclusion is to hold, but did not add anything that assists with this problem.

It is testified that it was from Egypt that Thales acquired the rudiments of geometry. However, the evidence is that the Egyptian skills were in orientation, measurement, and calculation. Thales’s unique ability was with the characteristics of lines, angles and circles. He recognized, noticed and apprehended certain principles which he probably ‘proved’ through repeated demonstration.



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