Greek philosophy, which occupied six centuries before, and six after, Christ, may be divided into four periods, corresponding with the succession of the principal lines of research (1) From Thales of Miletus to Socrates (seventh to fifth centuries B.C. — preoccupied with cosmology) (2) Socrates, Plato, and Aristotle (fifth to fourth centuries B.C. — psychology); (3) From the death of Aristotle to the rise of neo-Platonism (end of the fourth century B.C. to third century after Christ — moral philosophy); (4) neo-Platonic School (from the third century after Christ, or, including the systems of the forerunners of neo-Platonism, from the first century after Christ, to the end of Greek philosophy in the seventh century — mysticism).
The pre-Socratic philosophers either seek for the stable basis of things — which is water, for Thales of Miletus; air, for Anaximenes of Miletus; air endowed with intelligence, for Diogenes of Apollonia; number, for Pythagoras (sixth century B.C.); abstract and immovable being, for the Eleatics — or they study that which changes: while Parmenides and the Eleatics assert that everything is, and nothing changes or becomes. Heraclitus (about 535-475) holds that everything becomes, and nothing is unchangeable. Democritus (fifth century) reduces all beings to groups of atoms in motion, and this movement, according to Anaxagoras, has for its cause an intelligent being.
When the Sophists (Protagoras, Gorgias) had demonstrated the insufficiency of these cosmologies, Socrates (470-399) brought philosophical investigation to bear on man himself, studying man chiefly from the moral point of view. From the presence in us of abstract ideas Plato (427-347) deduced the existence of a world of supersensible realities or ideas, of which the visible world is but a pale reflection. These ideas, which the soul in an earlier life contemplated, are now, because of its union with the body, but faintly perceived. Aristotle (384-322), on the contrary, shows that the real dwells in the objects of sense. The theory of act and potentiality, of form and matter, is a new solution of the relations between the permanent and the changing. His psychology, founded upon the principle of the unity of man and the substantial union of soul and body, is a creation of genius. And as much may be said of his logic.
After Aristotle (end of the fourth Century B.C.) four schools are in evidence: Stoic, Epicurean, Platonic, and Aristotelean. The Stoics (Zeno of Citium, Cleanthes, Chrysippus), like the Epicureans, make speculation subordinate to the quest of happiness, and the two schools, in spite of their divergencies, both consider happiness to be ataraxia or absence of sorrow and preoccupation. The teachings of both on nature (Dynamistic Monism with the Stoics, and Pluralistic Mechanism with the Epicureans) are only a prologue to their moral philosophy. After the latter half of the second century B.C. we perceive reciprocal infiltrations between the various schools. This issues in Eclecticism. Seneca (first century B.C.) and Cicero (106-43 B.C.) are attached to Eclecticism with a Stoic basis; two great commentators of Aristotle, Andronicus of Rhodes (first century B.C.) and Alexander of Aphrodisia about 200), affect a Peripatetic Eclecticism. Parallel with Eclecticism runs a current of Scepticism (Ænesidemus, end of first century B.C., and Sextus Empiricus, second century A.D.).
In the first century B.C. Alexandria had become the capital of Greek intellectual life. Mystical and theurgic tendencies, born of a longing for the ideal and the beyond, began to appear in a current of Greek philosophy which originated in a restoration of Pythagorism and its alliance with Platonism (Plutarch of Chieronea, first century B.C.; Apuleius of Madaura; Numenius, about 160 and others), and still more in the Graeco-Judaic philosophy of Philo the Jew (30 B.C. to A.D. 50). But the dominance of these tendencies is more apparent in neo-Platonism. The most brilliant thinker of the neo-Platonic series is Plotinus (A.D. 20-70). In his “Enneads” he traces the paths which lead the soul to the One, and establishes, in keeping with his mysticism, an emanationist metaphysical system. Porphyry of Tyre (232-304), a disciple of Plotinus, popularizes his teaching, emphasizes its religious bearing, and makes Aristotle’s “Organon” the introduction to neo-Platonic philosophy. Later on, neo-Platonism, emphasizing its religious features, placed itself, with Jamblichus, at the service of the pagan pantheon which growing Christianity was ruining on all sides, or again, as with Themistius at Constantinople (fourth century), Proclus and Simplicius at Athens (fifth century), and Ammonius at Alexandria, it took an Encyclopedic turn. With Ammonius and John Philoponus (sixth century) the neo-Platonic School of Alexandria developed in the direction of Christianity.
In the closing years of the second century and, still more, in the third century, the philosophy of the Fathers of the Church was developed. It was born in a civilization dominated by Greek ideas, chiefly neo-Platonic, and on this side its mode of thought is still the ancient. Still, if some, like St. Augustine, attach the greatest value to the neo-Platonic teachings, it must not be forgotten that the Monist or Pantheistic and Emanationist ideas, which have been accentuated by the successors of Plotinus, are carefully replaced by the theory of creation and the substantial distinction of beings; in this respect a new spirit animates Patristic philosophy. It was developed, too, as an auxiliary of the dogmatic system which the Fathers were to establish. In the third century the great representatives of the Christian School of Alexandria are Clement of Alexandria and Origen. After them Gregory of Nyssa, Gregory of Nazianzus, St. Ambrose, and, above all, St. Augustine (354-430) appear. St. Augustine gathers up the intellectual treasures of the ancient world, and is one of the principal intermediaries for their transmission to the modern world. In its definitive form Augustinism is a fusion of intellectualism and mysticism, with a study of God as the centre of interest. In the fifth century, pseudo-Dionysius perpetuates many a neo-Platonic doctrine adapted to Christianity, and his writings exercise a powerful influence in the Middle Ages.
The philosophy of the Middle Ages developed simultaneously in the West, at Byzantium, and in divers Eastern centres; but the Western philosophy is the most important. It built itself up with great effort on the ruins of barbarism: until the twelfth century, nothing was known of Aristotle, except some treatises on logic, or of Plato, except a few dialogues. Gradually, problems arose, and, foremost, in importance, the question of universals in the ninth, tenth, and eleventh centuries. St. Anselm (1033-1109) made a first attempt at systematizing Scholastic philosophy, and developed a theodicy. But as early as the ninth century an anti-Scholastic philosophy had arisen with Eriugena who revived the neo-Platonic Monism.
In the twelfth century Scholasticism formulated new anti-Realist doctrines with Adelard of Bath, Gauthier de Mortagne, and, above all, Abelard and Gilbert de la Porrée, whilst extreme Realism took shape in the schools of Chartres. John of Salisbury and Alain de Lille, in the twelfth century, are the co-ordinating minds that indicate the maturity of Scholastic thought. The latter of these waged a campaign against the Pantheism of David of Dinant and the Epicureanism of the Albigenses — the two most important forms of anti-Scholastic philosophy. At Byzantium, Greek philosophy held its ground throughout the Middle Ages, and kept apart from the movement of Western ideas. The same is true of the Syrians and Arabs.
But at the end of the twelfth century the Arabic and Byzantine movement entered into relation with Western thought, and effected, to the profit of the latter, the brilliant philosophical revival of the thirteenth century. This was due, in the first place, to the creation of the University of Paris; next, to the foundation of the Dominican and Franciscan orders; lastly, to the introduction of Arabic and Latin translations of Aristotle and the ancient authors. At the same period the works of Avicenna and Averroes became known at Paris. A pleiad of brilliant names fills the thirteenth century — Alexander of Hales, St. Bonaventure, Bl. Albertus Magnus, St. Thomas Aquinas, Godfrey of Fontaines, Henry of Ghent, Giles of Rome, and Duns Scotus — bring Scholastic synthesis to perfection. They all wage war on Latin Averroism and anti-Scholasticism, defended in the schools of Paris by Siger of Brabant. Roger Bacon, Lully, and a group of neo-Platonists occupy a place apart in this century, which is completely filled by remarkable figures.