Again, all the sciences take for granted that man can know something about the universe. What is this knowledge? Is it really valid, as it claims to be? -Is there any way of ascertain ing its validity? A scientific treatment of these questions is called epistemology, or the theory of knowledge (q.v.), and forms a second philo sophical discipline. Still again, can any light be thrown on the ultimate nature of the reality either of the external world or of the mind? This is the problem of metaphysics (q.v.). Formerly logic. psyeholog,y, and ethics were considered as branches of philosophy, but the modern tendency is to establish them as separate sciences. This is accomplished. however. only in a restricted sense. At bottom the question whether this or that dis cipline is a science or a branch of philosophy is of no great importance, when it is recognized that there is no fundamental difference in method between science and philosophy, the only differ ence being one of degree, not of kind. The more abstract any systematic knowledge is, the nar rower the field it appropriates to itself; the more it neglects other szibjects lying outside of that field, so much the more properly is it called special science. (in the contrary, the more com prehensive such knowledge is, the more philo sophical it is; for philosophy is an attempt as far as possible to know the world as a whole.
The history of Euroliean philosophy is commonly divided into three main epochs, each characterized by a unique development. although at the same time the earlier periods furnish preface and starting-point for those which suc ceed them. so that the whole development is con tinuous. The first of these periods is that of an cient or Greek philosophy (q.v.). It extends from the earliest formulation; of philosophic con ceptions among the Ionian philosophers and con tinues into the Christian Era, throughout the dominance of pagani-an. While shared by other nations of antiquity besides the Greeks, as notably by certain Romans, Jews. and Grxco-Egyptians. it, received its form and lasting bent from the Greek mind, and its later influence proceeds al most exclusively from Athenian culture. The second period is that of media•yal philosophy. or scholasticism (q.v.). It had its rise in the theological speculations of the early Church Fathers. usually educated in Greek thought, and was continued and developed throughout medie val limes as the characteristic philosophy of Catholic Christianity. it had its culmination in the great synthesis of Thomas Aquinas. but has never ceased to be the traditional philosophy of the Church. and in modern times, as ueo-seholas ticism, is the subject of an energetic revival.
During the scholastic period in Chrigtion Europe the tradition of Greek philosophy was kept alive by the Arabs in Spain (see ARAnte LANGUAGE AND LtTtataTunm ). among whom were many im portant philosophical thinkers. The third period is kimwn as the period of modern philosophy, and is commonly dated from the 1Zenaissance and more particularly from the advocacy of em pirical methods of investigation by Bacon and of appeal to immediate reason or intuition by Descartes.
With the revival of learning there was also a revival of all the importan;, philosophical sys t.m. of antiquity. Phitonism flourished in the Academy of Florence and found its most impor tant advocate in Ficino. Neu-Platonism blended with Neo-Pythagoreanis) which was represented by Pico della Mirando .1. Aristotelianism re newed its vigor in the two -rival schools of Averroism and Alexandrism, and among the Prot estants in Melanchthon. Skepticism was defended by Montaigne, Sanchez, and Ch a rron ; eclecticism, virulent in its opposition to propagated by Valhi. Vives„ de In Rami7-e, and Zwingli, while scholasticism found its champion in Suarez. Natural philosophy, a rehabilitation of one phase of Aristotle's system, but blended with Nco-Platonie mysticism, was developed by Cardano. Campanella, and especially by Bruno. Paracelsus and name elaborated the mystical side of this line of thought. while Galileo and Kepler remained more true to an empirical study of nature, and formed the stirting point of a mechanistic and materialistic conception of the universe. in Kepler, however. with his insistence on the harmony of the world order, we have a distinctively Pythagorean touch. Political phil osophy took an entirely new start with Machia velli, who threw aside the work of Aristotle and the authority of the Church, and worked out an anti-moral theory•in which the State and civic freedom were all-important. But though the philosophers of the Renaissance thus bridged over the chasm that separated the ancient from the modern world of thought, it was reserved for Bacon and Descartes to lay the foundations of modern philosophy in empiricism 011 the one hand and in rationalism on the other; and the real problem of modern philosophy has been to rear a superstructure which should rest on both these foundations. As was to be expected, the foundations had first to he laid independently, and as a result we have two great modern schools. each equally one-sided. but each equally standing for a fundamental truth—the one, the truth that all secure knowledge rests on ex perience; the other, the truth that in all satis factory knowledge the demands of reason are met.