Neo-Platonism when first constituted had incorporated a large Aristotelian element, the expulsion of which had been accomplished by its last great master, Proclus; and Christendom took over metaphysics under what seemed a Platonic form—the more welcome as Plato passed for giving its creeds the independent support of pure reason. This support extended beyond a future life and went down to the deepest mysteries of revealed faith. For, according to the Platonic doctrine of ideas, it was quite in order that there should be a divine unity existing independently of the three divine persons composing it; that the idea of humanity should be combined with one of these persons; and that the same idea, being both one with and distinct from Adam, should involve all mankind in the guilt of his transgression. Thus the Church started with a strong prejudice in favour of Plato which continued to operate for many centuries, although the first great schoolman, John Scotus Eriugena (810-877), incurred a condemnation for heresy by adopting the pantheistic metaphysics of Neo-Platonism.
As the Platonic doctrine of ideas came to life again in the realism, as it was called, of scholastic philosophy, so the conflicting view of his old opponent Aristotle was revived under the form of conceptualism. According to this theory the genera and species of the objective world correspond to real and permanent distinctions in the nature of things; but, apart from the conceptions by which they are represented in the intellect of God and man, those distinctions have no separate existence. Aristotle's philosophy was first brought into Europe by the Mohammedan conquerors of Spain, which became an important centre of learning in the earlier Middle Ages. Not a few Christian scholars went there to
study. Latin translations were made from Arabic versions of Aristotle, and in this way his doctrines became more widely known to the lecture-rooms of the Catholic world. But their derivation from infidel sources roused a prejudice against them, still further heightened by the circumstance that an Arabian commentator, Averroes, had interpreted the theology of the Metaphysics in a pantheistic sense. And on any sincere reading Aristotle denied the soul's immortality which Plato had upheld. Accordingly, all through the twelfth century Platonism still dominated religious thought, and even so late as the early thirteenth century the study of Aristotle was still condemned by the Church.
Nevertheless a great revolution was already in progress. As a result of the capture of Constantinople by the Crusaders in A.D. 1204 the Greek manuscripts of Aristotle's writings were brought to Paris, and at a subsequent period they were translated into Latin under the direction of St. Thomas Aquinas, the ablest of the schoolmen, who so manipulated the Peripatetic philosophy as to convert it from a battering-ram into a buttress of Catholic theology—a position still officially assigned to it at the present day. Aristotelianism, however, did not reign without a rival even in the later Middle Ages. Aquinas was a Dominican; and the jealousy of the competing Franciscan Order found expression in maintaining a certain tradition of Platonism, represented in different ways by Roger Bacon (1214-1294) and by Duns Scotus (1265-1308). In this connection we have to note the extraordinary fertility of the British islands in eminent thinkers during the Middle Ages. Besides the two last mentioned there is Eriugena ("born in Ireland"), John of Salisbury
(1115-1180), the first Humanist, William of Ockham, and Wycliffe, the first reformer—making six in all, a larger contribution than any other region of Europe, or indeed all the rest of Europe put together, has made to the stars of Scholasticism. This advantage is probably not due to any inherent genius for philosophy in the inhabitants of these islands, but to their relative immunity from war and to the political liberty that cannot but have been favourable to independent thought. Five out of the six were more or less inclined to Platonism, and their idealist or mystical tendencies were sometimes associated with the same practicality that distinguished their master. The sixth, commonly called Occam (died about 1349), is famous as the champion of Nominalism—that is, of the doctrine that genera and species have no real existence either in nature or in mind; there are only individuals more or less resembling one another. He is the author of the famous saying—the sole legacy of Scholasticism to common thought: "Entities ought not to be gratuitously multiplied" (entia non sunt præter necessitatem multiplicanda).
The capture of Constantinople by the Crusaders had led to Aristotle's triumph in the thirteenth century. Two hundred years later the conquering Ottoman advance on the same city was the immediate cause of his overthrow. For the Byzantine scholars who fled for help and refuge to Italy brought with them the manuscripts of Plato and Plotinus, and these soon became known to Western Europe through the Latin translations of Marsilio Ficino. On its literary side the Platonic revival fell in admirably with the Humanism to which the Schoolmen had long been intensely distasteful. And the religious movement that preceded
Luther's Reformation found a welcome ally in Neo-Platonic mysticism. At the same time the invention of printing, by opening the world of books to non-academic readers, vastly widened the possibilities of independent thought. And the Reformation, by discrediting the scholastic theology in Northern Europe, dealt another blow at the system with which it had been associated by Aquinas.
It has been supposed that the discovery of America and the circumnavigation of the globe contributed also to the impending philosophical revolution. But the true theory of the earth's figure formed the very foundation of Aristotle's cosmology, and was as well known to Dante as to ourselves. Made by a fervent Catholic, acting under the patronage of the Catholic queen par excellence, the discovery of Columbus increased the prestige of Catholicism by opening a new world to its missions and adding to the wealth of its supporters in the Old World.
The decisive blow to medieval ideas came from another quarter—from the Copernican astronomy. What the true theory of the earth's motion meant for philosophy has not always been rightly understood. It seems to be commonly supposed that the heliocentric system excited hostility because it degraded the earth from her proud position as centre of the universe. But the reverse is true. According to Aristotle and his scholastic followers, the centre of the universe is the lowest and least honourable, the circumference the highest and most distinguished position in it. And that is why earth, as the vilest of the four elements, tends to the centre; while fire, being the most precious, flies upward. Again, the incorruptible æther of which the heavens are composed shows its eternal character