XII.

THE PHYSICAL SYSTEM WITH RESPECT TO PHYSICS IN GENERAL. THE NATURE OF THIS SCIENCE.

IN the division of sciences, whether speculative or practical, we have to consider the proper object of each, not in itself, but as treated by them; and this constitutes the formal object. There are three classes of things that may be the object of speculative science: Firstly, those that are in matter and are considered as in matter; secondly, those that are considered as without matter, but cannot be otherwise than in matter; thirdly, those that transcend matter, either because they cannot be in matter, or because, though they are in matter, they may be separated from it. The first of these three classes gives the object of Physics, for that science deals with sensible things that are in matter and are contemplated as in matter. They are these : 1. corporeal substances without life; 2. those that merely vegetate; 3. those that are animate and sensitive, but not rational; 4. those that are sensitive, animate and rational. The second class gives us the object of Mathematics, a science that treats of quantity, either continuous, as it is in geometry, or discrete, as in arithmetic and algebra; for although quantity cannot naturally be without matter, nevertheless in those sciences, matter is considered by abstraction as apart from quantity. The third class (i.e., that which transcends matter, etc.) gives us the object of metaphysics, which treat of substances separated from matter, as God and spirits, or of things that may belong to matter, but are also found without it, such as the essence of Being, Substance, Actus, Potentia, Cause, effect, &c. Thus the three speculative sciences are Physics, Mathematics, and Metaphysics.

St. Thomas teaches this as follows "Because," he says, "the first book of Physics, which we purpose to expound, is the first book of natural science, we must first determine what is the matter and the subject of natural science. It must therefore be understood that, since all science is in the intellect, and things become actually intelligible by being in a certain manner abstracted from matter, things belong to different sciences in accordance with their different relations to matter. Again, since every science is acquired through demonstration, and the medium of demonstration is definition, it necessarily follows that sciences differ according to the diversity of their definitions. Be it known then, that there are some things whose being depends on matter, and which without matter cannot be defined. There are others that cannot be except in sensible matter; yet sensible matter does not come into their definition." . . . Of the former sort, he says, are all natural things, as man, for instance, or a stone. Of the latter are all mathematics -- as numbers, magnitude, figure. "But there are some things," he goes on to say, "that are independent of matter not only as to their being, but also in our conception of them, either because they never are in matter, as God and other separated substances, or because they are not always in matter, as substance, potentia, actus and Being, (Ens). Metaphysics treat of these; mathematics of those that depend on matter for their being, but not in the manner of conceiving them. Natural science, which is called physical, treats of those things that depend on sensible matter both in their being and in the manner of conceiving them. And since all that has matter is moveable, the consequence is that ens mobile is the subject of natural philosophy. For natural philosophy treats of natural things and natural things are those whose principle is nature: and nature is the principle of motion and of rest in that wherein it is. Natural science therefore treats of those things that have in themselves a principle of motion."^{1} The holy Doctor requires us to recognize in the object of Physics two properties. The former is that it can neither be nor be defined without matter. The latter (which belongs to the former) is its being endowed with an inner principle of activity or of nature. Hence the name of Physics, as we said before.

This will serve to show how vast is the domain of physics, embracing as its proper object every corporeal substance, or, in other words, the whole visible universe. Many, however, give to metaphysics that part which concerns living substances, viz., plants, brutes and man. According to this division, physical science is restricted to things without life, and is called general when treating of them as a whole, special as discussing the various species; chief among which are chemical physics, mechanical physics and astronomic physics. To go into all these would lengthen our work enormously without serving its purpose: and therefore we restrict ourselves to General Physics. Of the others, which are subordinate thereto, we shall show that their fundamental principles are not in opposition to the discoveries of modern science. And here we distinguish between science and scientists: for scientists often disagree among themselves on many important points, or confine themselves to exposition of facts, setting aside philosophical principles; or explain those facts by hypotheses that fit into their systems but are not grounded on certainty.

{1} Comm. Phys. Aris., Lib. i. Lect. i.