a) Formal object. - A basic problem in any science is to determine its formal object, its specific area or aspect of inquiry. Aristotle's conception of the formal object in natural philosophy is not only clear and precise but bids fair to endure in the future as in the past. The world of nature, he declares, is one of perpetual change, of inherent mutability. To appreciate the whole import of this observation, one must go back to the Greek Physicists, all of whom were struck by this unceasing alternation of ruin and renewal that marks the course of nature. Had not Heraclitus advertised that one could never bathe twice in the identical stream? That all nature was in constant flux? Aristotle, then, was stressing again what others had stressed before. A thing of nature is changeable in its very essence.
Consequently, the formal object of natural philosophy must incorporate this element of instability. To say that natural philosophy studies natural being is not enough; it lacks precision. We add this precision by saying that the formal object of natural philosophy is "being considered under this very aspect of changeableness." In Scholastic phrase it is mobile being, ens mobile, which comes to the same. Mobile means changeable, and being qua being is not changeable unless it is material, hence a being of nature.
St. Thomas speaks in the same vein; he writes: "The philosophy of nature, which is called Physics, treats those things which depend on matter, not only for existence but also in definition. And because everything that has matter is mobile, it follows that mobile being is the subject of the philosophy of nature." 3
In these words, which are of prime importance, St. Thomas not only defines the formal object of natural philosophy by "mobility," but traces this mobility to the material constitution of the beings which this branch of philosophy considers. A material thing is by that very fact a mobile or changeable being, whereas an immaterial being is essentially unchanging. Note, further, that in Aristotelian usage "mobile" and "movement" are taken most widely, denoting every possible kind of changeableness or change in the world of nature.
b) Divisions of natural philosophy. - Aristotle's physical doctrine can be divided into two major parts. The first, which corresponds to the eight books of the Physics, treats of mobile being in general. The other, which includes the rest of his physical works, studies particular kinds of movement and mobile being. This grouping follows a pattern; the sequence is from general observations to particular considerations. And this is as it should be, at least for the orderly presentation of any branch of knowledge.
But the order and object of treatment within each member of this bipartite division, especially within the second, are not so self-evident. Here, in fact, we are on debated, if not debatable, ground. Be that as it may, St. Thomas apportions natural philosophy as follows.
The study of mobile being in general falls under two principal headings: that of mobile being itself, which is studied in Physics, I-II, and that of motion, focal theme of the remaining books, III-VIII.
As for the particular kinds of motion and mobile being, these are studied in separate treatises, each centering round a principal type of change and mobile being. Thus, De Caelo (On the Heavens) treats of natural beings so far as they are subject to the primary species of motion, which is local motion. De Generatione et Corruptione (On Generation and Corruption) considers motion toward form (generation-corruption, alteration, growth-diminution), and also the primary mobile beings, the elements, but only according to their common transformations; their special transformations are the subject of the Meteorologica (Meteorology). Other treatises investigate so-called "mixed" mobile beings,' some of which are animate, some inanimate. Inanimate mixtures are the topic of De Mineralibus (On Minerals), animate mixtures the topic of De Anima (On the Soul) and its sequential studies, namely, the Parva Naturalia (Minor Natural Works) and a number of animal studies.5
Our study, in the main, is confined to motion and mobile being in general; which means it follows the broad outlines of the Physics, both in content and procedure. Of Books V and VI, however, we make only very selective examination, for these have mostly to do with special problems that may be put aside in an introductory study. Dismissed with naming, moreover, is Book VII, which appears to be an interpolation. Thus, treated in order are the following topics: