EUCLID, of Megara, a Greek philosopher, the founder of the Megarian school, was born in the latter half of the 5th century B.C., probably at Megara, though Gela in Sicily has also been named as his birth-place. He was one of the most devoted of the disciples of Socrates. If we may believe Aulus Gellius, such was his enthusiasm that, when a decree was passed forbidding the Megarians to enter Athens, he regularly visited his master by night in the disguise of a woman ; and he was one of the little band of intimate friends who had the privilege of listening to the hero's last dis-course. After his master's death, he withdrew, with a number of his fellow-disciples, to Megara ; and it has been conjectured, though there is no direct evidence, that this was the period of Plato's residence in Megara, of which indications appear in the Thecetetus. The fundamental prin-ciple of Euclid's philosophy was a combination of the Eleatic conception of Being—the One and All, and the Socratic conception of the Good. Being is immaterial and unchangeable, and is identical with the Good, which is the same as God, as Reason, and (following the Socratic doctrine) as Wisdom, and which alone truly exists. Thus the existence of evil was denied; and the main object of the Megarian, as it was of the Éleatic dialectic, was to prove the conceptions of division, number, becoming, motion, and possibility to be self-contradictory and false. With Plato, Euclid taught that sense has cognizance of the changeable and unreal only, while thought penetrates to unchangeable Being, to the Good. The Megarian school prided itself first of all upon its dialectic. Euclid's dialectic differed greatly from that of his master Socrates, in marked contrast to whom he repudiated the principle of analogical reasoning as unsound. His favourite method of attacking an oppon-ent was by the reductio ad absurdum, which was also a favourite method with his followers, whose arguments degenerated into trivial sophisms, which laid them frequently open to an attack with their own weapon, and which earned for them the contemptuous name of the 'Epto-rtKoi or " wranglers." Of Euclid's followers the chief were Eubutides, who taught Demosthenes, wrote against Aristotle, and invented several trifling but ingenious paradoxes, of which the most famous is the Sorites ; Diodorus Chronus, the author of certain arguments to prove the impossibility of motion ; Philo; and, most famous of all, Stilpo, who was distinguished by the attractiveness of his lectures.

Our knowledge of Euclid's philosophy is borrowed from scattered passages in Plato, and from Diogenes Laertius. Sec Zeller, Socrates and the Socratic Schools ; Dyeck, De Megaricorum Doctrina (Bonn, 1827) ; Mallet, Histoire de i'Ecole de Megare (Paris, 1845) ; Eitter, Ueher die Philosophic der Meg. Scliule; Prantl, Geschichtc cler Zogitc, ]., 33 ; Henne, L'Ecole de Megare (Paris, 1843).