THALES OF MILETUS B.c.), Greek physical philosopher, son of Examyus and Cleobuline, is said to have been of Phoenician extraction, but was more probably a native Milesian of noble birth. Thales was certainly a Greek and not a Phoenician. He was chief of the seven "wise men" of Greece; and in later times amongst the ancients his fame was remarkable. This name was given on account of practical ability; and Thales had been occupied with civil affairs. The advice which he gave to his fellow-countrymen "before Ionia was ruined"—"that the Ionians should constitute one general council in Teos, as the most central of the twelve cities, and that the remaining cities should nevertheless he governed as independent states" (Herod. i. 17o) —is noteworthy. The appellation "wise man" was conferred on him not only for his political sagacity, but also for his scientific eminence (Plut. Solon, c. 3).
He became famous by his prediction of the eclipse of the sun of May 28, 585 B.C. Herodotus's account of it (i. 74) contains two statements:—(i) the fact that the eclipse did actually take place during a battle between the Medes and the Lydians, that it was a total eclipse, that it caused a cessation of hostilities and led to a lasting peace between the contending nations; (2) that Thales had foretold the eclipse to the Ionians, and fixed the year in which it actually did take place. Various dates—ranging from 625 B.C. to 583 B.C.—have been assigned by different chronologists to this eclipse; but, since the investigations of Airy', Hind', and the date determined by them (May 28, 585 B.c.) has been gen erally accepted (for later authorities see ECLIPSE and ASTRON OMY). This date agrees nearly with that given by Pliny.
Thales's fame amongst the ancients must have been largely due to this achievement. Thales seems to have left no works (but see Diog. Laer. i. 23). Many anecdotes are related of him, from some of which it would appear that he was engaged in trade (Plutarch, Solon, c. 2). Of the fact that Thales visited Egypt, and there became acquainted with geometry, there is abundant evidence. Hieronymus of Rhodes (ap. Diog. Laer. i. 27) says,
"he never had any teacher except during the time when he went to Egypt and associated with the But the important feature of Thales's work was that he founded the geometry of lines, which was essentially abstract in its char acter. The Egyptian priests only had the geometry of surfaces, a sketch of that of solids, i.e., a geometry consisting of some simple quadratures and elementary cubatures, obtained empiri cally. Thales introduced abstract geometry, the object of which is to establish precise relations between the different parts of a figure, so that some of them could be definitely found by means of others. This was a phenomenon quite new in the world.
The following discoveries in geometry are attributed to Thales: —(I) the circle is bisected by its diameter (Procl. op. cit. p. 157) ; (2) the angles at the base of an isosceles triangle are equal (Id. p. 25o) ; (3) when two straight lines intersect the vertically op posite angles are equal (Id. p. 299) ; (4) the angle in a semi circle is a right angle', (5)' the theorem Euclid i. 26 (Eudemus, Procl. op. cit. p. 352). Two applications to practical problems are also attributed to him :—(I) the determination of the distance of a ship at sea; (2) the determination of the height of a pyramid by means of the length of its shadow : the shadow was measured at the hour of the day when a man's shadow is the same length as himself'. According to Plutarch (Sept. Sap. Conviv. 2), Thales must have known Euclid vi. 4, but without the restriction as to the hour of the day. Further, we learn that he perfected the '"On the Eclipses of Agathocles, Thales and Xerxes," Phil. Trans. vol. cxliii., p. 179 seq., 1853.
Untersuchungen der wichtigeren Finsternisse, etc., P. 57, 1853.