Plato's classification was taken up by his school (the Academy), but it was not long in yielding to the influence of Aristotle's more complete division and according a place to logic. Following the inspirations of the old Academics, the Stoics divided philosophy into physics (the study of the real), logic (the study of the structure of science), and morals (the study of moral acts). This classification was perpetuated by the neo-Platonists, who transmitted it to the Fathers of the Church, and through them to the Middle Ages.
(2) Aristotle, Plato's illustrious disciple, the most didactic, and at the same time the most synthetic, mind of the Greek world, drew up a remarkable scheme of the divisions of philosophy. The philosophical sciences are divided into theoretic, practical, and poetic, according as their scope is pure speculative knowledge, or conduct (πράξις), or external production (ποίησις). Theoretic philosophy comprises: (a) physics, or the study of corporeal things which are subject to change (άχώριστα μέν άλλ' ούκ άκίνητα) (b) mathematics, or the study of extension, i.e., of a corporeal property not subject to change and considered, by abstraction, apart from matter (άκίνητα μέν ού χώριστα δ' ίσως, άλλ' ώς έν ύλη); (c) metaphysics, called theology, or first philosophy, i. e. the study of being in its unchangeable and (whether naturally or by abstraction) incorporeal determinations (χώριστα καί άκίνητα). Practical philosophy comprises ethics, economics, and politics, the second of these three often merging into the last. Poetic philosophy is concerned in general with the external works conceived by human intelligence. To these may conveniently be added logic, the vestibule of philosophy, which Aristotle studied at length, and of which he may be called the creator.
To metaphysics Aristotle rightly accords the place of honour in the grouping of philosophical studies. He calls it “first philosophy”. His classification was taken up by the Peripatetic School and was famous throughout antiquity; it was eclipsed by the Platonic classification during the Alexandrine period, but it reappeared during the Middle Ages.
B. In the Middle Ages.—Though the division of philosophy into its branches is not uniform in the first period of the Middle Ages in the West, i. e. down to the end of the twelfth century, the classifications of this period are mostly akin to the Platonic division into logic, ethics, and physics. Aristotle's classification of the theoretic sciences, though made known by Boethius, exerted no influence for the reason that in the early Middle Ages the West knew nothing of Aristotle except his works on logic and some fragments of his speculative philosophy (see section V below). It should be added here that philosophy, reduced at first to dialectic, or logic, and placed as such in the Trivium, was not long in setting itself above the liberal arts.
The Arab philosophers of the twelfth century (Avicenna, Averroes) accepted the Aristotelean classification, and when their works—particularly their translations of Aristotle's great original treatises—penetrated into the West, the Aristotelean division definitively took its place there. Its coming is heralded by Gundissalinus (see section XII), one of the Toletan translators of Aristotle, and author of a treatise, “De divisione philosophiae”, which was imitated by Michael Scott and Robert Kilwardby. St. Thomas did no more than adopt it and give it a precise scientific form. Later on we shall see that, conformably with the medieval notion of sapientia, to each part of philosophy corresponds the preliminary study of a group of special sciences. The general scheme of the division of philosophy in the thirteenth century, with St. Thomas's commentary on it, is as follows:
There are as many parts of philosophy as there are distinct domains in the order submitted to the philosopher's reflection. Now there is an order which the intelligence does not form but only considers; such is the order realized in nature. Another order, the practical, is formed either by the acts of our intelligence or by the acts of our will, or by the application of those acts to external things in the arts: hence the division of practical philosophy into logic, moral philosophy, and æsthetics, or the philosophy of the arts (“Ad philosophiam naturalem pertinet considerare ordinem rerum quem ratio humana considerat sed non facit; ita quod sub naturali philosophia comprehendamus et metaphysicam. Ordo autem quem ratio considerando facit in proprio actu, pertinet ad rationalem philosophiam, cujus est considerare ordinem partium orationis ad invicem et ordinem principiorum ad invicem et ad conclusiones. Ordo autem actionum voluntariarum pertinet ad considerationem moralis philosophiæ. Ordo autem quem ratio considerando facit in rebus exterioribus per rationem humanam pertinet ad artes mechanicas.” To natural philosophy pertains the consideration of the order of things which human reason considers but does not create—just as we include metaphysics also under natural philosophy. But the order which reason creates of its own act by consideration pertains to rational philosophy, the office of which is to consider the order of the parts of speech with reference to one another and the order of the principles with reference to one another and to the conclusions. The order of voluntary actions pertains to the consideration of moral philosophy, while the order which the reason creates in external things through the human reason pertains to the mechanical arts.—In “X Ethic. ad Nic.”, I, lect. i). The philosophy of nature, or speculative philosophy, is divided into metaphysics, mathematics, and physics, according to the three stages traversed by the intelligence in its effort to attain a synthetic comprehension of the universal order, by abstracting from movement (physics), intelligible quantity (mathematics), being (metaphysics) (In lib. Boeth. de Trinitate, Q. v., a. 1). In this classification it is to be noted that, man being one element of the world of sense, psychology ranks as a part of physics.
C. In Modern Philosophy—The Scholastic classification may be said, generally speaking, to have lasted, with some exceptions, until the seventeenth century. Beginning with Descartes, we find a multitude of classifications arising, differing in the principles which inspire them. Kant, for instance, distinguishes metaphysics, moral philosophy, religion, and anthropology. The most widely accepted scheme, that which still governs the division of the branches of philosophy in teaching, is due to Wolff (1679-1755), a disciple of Leibniz, who has been called the educator of Germany in the eighteenth century. This scheme is as follows:
(2) Speculative Philosophy
- Ontology, or General Metaphysics.
- Special Metaphysics.
(3) Practical Philosophy.Wolff broke the ties binding the particular sciences to philosophy, and placed them by themselves; in his view philosophy must remain purely rational. It is easy to see that the members of Wolff's scheme are found in the Aristotelean classification, wherein theodicy is a chapter of metaphysics and psychology a chapter of physics. It may even be said that the Greek classification is better than Wolff's in regard to speculative philosophy, where the ancients were guided by the formal object of the study—i. e. by