Cursus Philosophicus Thomisticus, Prima Pars Philosophiae Naturalis

Philosophy, if it is taken in the whole breadth of its signification, encompasses all science; for philosophy is said to be, as it were, the "love of science" or "friendship of science," by which account human wisdom is called philosophy by St. Thomas in the preface of the Ethics. And thus philosophy is a generic science encompassing under itself every naturally acquirable science, especially the speculative, the love of which is properly the love of science, because it is loved only for the sake of knowing.

A threefold genus of this speculative science is enumerated by the Philosopher¹ and St. Thomas in Metaphysics XI, lect. 7 and Opusculum 70, q. 5:² namely, natural science, mathematics, and metaphysics. Logic is also placed reductively under it, which deals not with things to be speculated upon, but with the very instruments of speculation. These sciences differ according to a diverse degree of knowability, as we have said in the books of the Posterior Analytics, q. 27.³ For natural science treats of things subject to motion and sensible change, and therefore it abstracts only from singular matter. Mathematics deals with quantity, insofar as it abstracts from motion and from matter clothed with sensible accidents. Metaphysics deals with being as such, insofar as it abstracts from all matter, both sensible and intelligible—that is, even from prime matter according to itself, which is perceived by the intellect alone.

Philosophy taken in this breadth is defined variously by authors. For some define philosophy from the etymology of the name, which is the "love of wisdom"; others from the mode of proceeding, which is the "knowledge of things as they are," that is, through their causes. Others, who define philosophy from the matter which it embraces, say that it is the "knowledge of human and divine things." Lastly others, who define philosophy from the effect to which it leads us, have said that it is the "contemplation of death," i.e., separating man from temporal and corporeal passions.

At present, we do not treat of philosophy in its total breadth. Rather, having left aside mathematics and metaphysics, we treat only of natural science, whose object comprehends every natural being, i.e., every mobile being according to some corporeal or extended motion, for purely spiritual motions do not pertain to this genus of science.

In considering this object, Aristotle proceeded in the highest order, dividing this science into four parts:

The first deals with mobile being in common, for one proceeds scientifically from the more common to the less common. And this part is concluded in the eight books which they call the Physics or the "Lectures on Nature."

The remaining three parts deal with mobile being in particular. And according to the three species of motion, they are distinguished into three genera of mobile things, as St. Thomas observes in the preface of the books On Generation. The first is local motion; the second, alteration, which is ordered toward generation; the third, augmentation, which is proper to a living or animated thing. And according to the first motion, simple bodies are considered, from which this universe is constructed. And thus after the books of the Physics, the Philosopher treats of the heavens and the world,⁴ that is, of all simple bodies, insofar as they are coordinated and situated in this universe through local motion. For from these the universe especially consists; for mixed things have their place according to the mode of the element predominating in them. According to the motion of alteration, however, which is ordered toward generation, he treats of corruptible bodies, not insofar as they are coordinated by position in the universe, but insofar as they are subject to generation and corruption, both substantial and accidental. Thus the third part of philosophy deals with generation.⁵ Finally, according to the third genus of motion, namely augmentation and nutrition, which is proper to the living, he treats of every grade of life or of the soul which provides it, and he ascends even to the grade of intellective life, which is found in our soul; and of this he treats in the books On the Soul. And thus through these four parts the whole of natural science is divided.