The Eleatics, Parmenides and Zeno, had foreshadowed in the fifth century the division of philosophy into two branches; but Plato and his pupil Aristotle (in the fourth century B.C.) succeeded in gaining general acceptance for this dualism and antithesis of physics and metaphysics. Physics devoted itself on the ground of experience to the study of the phenomena of things, leaving their real essences (or noumena) that lay behind the phenomena to metaphysics. These inner essences are transcendental and inaccessible to empirical research; they form the metaphysical world of eternal ideas, which is independent of the real world, and has its highest unity in God, as the Absolute. The soul, an eternal idea that dwells for a time in the passing human body, is immortal. This consistent dualism of Plato's system, with its sharp antithesis of this world and the next, of body and soul, of world and God, is its chief characteristic. It became all the more influential when Plato's pupil Aristotle blended it with his empirical metaphysics, based on ample scientific experience, and pointed out the idea in the entelechy, or purposively acting principle, of every being; and especially when Christianity (three hundred years afterwards) found in this dualism a welcome philosophic support of its own transcendental tendency. In the course of the thousand years which historians call the Middle Ages, and which are usually dated from the fall of the Roman Empire (476) to the discovery of America (1492), the superstition of civilized races reached its highest development. The authority of Aristotle was paramount in philosophy; it was used by the dominant Church for its own purposes. But the influence of the Christian faith, with all the gay coloring which the fairy-tales of the Bible added to its structure of dogmas, was seen much more in practical life. In the foreground of belief were the three central dogmas of metaphysics, to which Plato had first given complete expression--the personal God as creator of the world, the immortality of the soul, and the freedom of the human will. As Christianity laid the greatest theoretical stress on the first two dogmas and the greatest practical stress on the third, metaphysical dualism soon prevailed on all sides. Especially inimical to scientific inquiry was the Christian contempt of nature and its belittlement of earthly life in view of the eternal life to come. As long as the light of philosophical criticism in any form was extinguished, the flower-garden of religious poetry flourished exceedingly and the idea of miracle was taken as self-evident. We know what the practical result of this superstition was from the ghastly history of the Middle Ages, with its Inquisition, religious wars, instruments of torture, and drowning of witches. In the face of the current enthusiasm for the romantic side of mediævalism, the Crusades and Church art, we cannot lay too much stress on these dark and bloody pages of its chronicles.