22 THE HISTORY OF MEDIEVAL EUROPE and harmony in the universe, Democritus, who first taught that the world is made up of atoms, Socrates, and Plato, to the late schools of thinkers called Stoics, Epicureans, and Neo-Platonists, of whom we shall have to speak again later. Of all Greek philosophers Aristotle was to be the most influential in the Middle Ages. He had profited by the teaching: of Plato, just as Plato had been the Aristotle ' ,. . , & r „ J , , . . . disciple of Socrates; but his own teaching was very different from the Platonic philosophy. Plato was a poetical idealist; Aristotle was more systematic and scien- tific. His History of Animals collected and classified a large amount of zoological data; his Poetics discussed various forms of literature and is our first fundamental work of literary criticism and theory; his Politics summarized the different forms of government existing in his day. More theoretical were his writings on physics, metaphysics, and ethics, but here too he dissented from Plato in many im- portant respects. Several of his treatises were devoted to psychological subjects; and in his works on logic he laid down sound rules which have been observed in the art of reasoning ever since. Aristotle was for a time the tutor of a young conqueror who was to change the map and civilization of the east- Alexander ern Mediterranean world. Alexander the Great, the Great King of Macedon, 336-323 B.C., finishing the work which his father Philip had prepared and begun, con- quered the world from the Balkans to Egypt and from the Greek peninsula to the frontier of India. Into this Oriental world, and especially into that portion of it which the Roman Empire later included, was now introduced Hellenic culture, which fused with what was left of the old cultures of Egypt, Babylon, and Assyria into a civilization termed "Hellenistic." At Alexandria in Egypt, named after and founded by Alexander, was developed the largest library in the ancient world, a zoological park and gardens to encour- age further investigations like Aristotle's History of Animals, and a learned society of librarians, editors, literary critics,
