a) The study of nature, of the physical universe, represents the most highly developed part of Aristotle's philosophy. It is certainly the part to which he devoted his most constant effort. Yet, so many and so great have been the advances of the physical sciences that a modern follower of Aristotelian thought is confronted with a very difficult problem. Briefly it is this.
In Aristotle's view, physics or natural philosophy is the third branch of speculative philosophy, the other two being metaphysics and mathematics. This division is based on differences incident to the object of knowledge. An object can be considered in progressive degrees of separation from matter - or, as Scholastics were more usually to say, according to various "degrees of abstraction." Applied to the physicist or inquirer of nature, this means he studies "the being of nature" but considers it in abstraction of individual characteristics. So, to take a well-worn example, the biologist does not study "this flesh" or "this bone" for what is individual to it, but he tries to discover what flesh or bone has generally.
As St. Thomas was later to explain it, on this level of inquiry abstraction is made from individual matter, a materia individuali, but sensible matter, materia sensibilis, is retained. Hence, this order of knowledge includes the properties reached by sense; it includes, for example, the color and sound and touch of things, but these properties are considered from their common, and not individual, aspects.
On this methodological foundation Aristotle erected his remarkable system of the physical universe. This system not only boasted a sturdy structure; it displayed a mastery of detail to match its architectonic triumph, and for near on two thousand years it commanded the scientific horizon. In the seventeenth century, however, a new dawn of scientific progress opened. Empirical search was pursued as never before, and the mathematical technique was pressed into the pursuit. The combination proved most successful. Soon there was amassed a new store of knowledge not less impressive than the older accumulation and far outdoing it in practicality. Science as we know it had been born, and once ushered into the world, its growth was phenomenal.
Not only was this scientific revolution achieved by methods that were new and, apparently, the complete opposite of the old, but the whole movement had the air of a revolt against the traditional order of science. And here lies the crux of the problem under consideration, in the alleged rift between the old science of nature and the new. For, what we now have is not one but two integral formulations of physical reality, both purporting to be true, yet one picturing it far differently from the other. Is it possible to reconcile these two versions of what appears to be the same reality, the same world of nature? We believe it is possible, provided that both recognize their limitations. Specifically, the Aristotelian philosophy of nature needs to divest itself of certain nonessentials, which are hangovers from an outdated science and do not affect its substance. Modern science, on the other hand, must forego the claim of being the highest wisdom, the last word on reality, a claim it can make by usurpation only.
b) The conditions we have suggested for resolving the "opposition" between the old and the new science of nature go to the heart of the problem. They assume - and this is fundamental - that facts and events of nature can be studied from two different points of view.
One may seek what is most basic in nature, its most universal characteristics and properties, relying for this inquiry on the evidence of ordinary experience. The questions asked are universal in scope. For example, what are the conditions underlying all change and movement? What are the ultimate principles of nature - atoms, elements, matter and form, or whatever else? This is the province of the philosophy of nature, and here Aristotle can still be taken as a sure guide.
But one may also set his sights more narrowly, limiting his inquiry to the particular circumstances of particular facts and events, such as the fall of bodies, the workings of magnetic forces, the phenomenon of evaporation, and countless others. This level of investigation, in which phenomena of the kind mentioned are subjected to properly scientific observation and measurement, corresponds to the science of nature. In this the advantage is all to the moderns.
The difference between these two approaches has been enunciated by Jacques Maritain. The philosophy of nature, he observes, does not disregard the objects perceived by sense (objects corresponding to the first degree of abstraction), but it explains them by principles that are broadly speaking ontological or metaphysical; for it employs such notions as corporeal substance, quality, active and passive potency, material and formal cause, and others of similarly "ontological" content. The sciences of nature, on the other hand, generally stick to more concrete notions. Theirs are the concepts of what is physically measurable, of things that lend themselves to verification by experience. And when the sciences go a step further, they do not resort to ontological but to mathematical principles, which fall short of the ontological degree of abstraction.
All which is by way of saying that our concepts of phenomenal nature, of observable facts and events, can be resolved in two ways: one, to quote Maritain, "is the upward resolution toward intelligible (as compared with sensible) being. In this process the sensible object is not lost sight of, but its presence is not focal. It lies in the background, where it continues to minister to intelligible being, in which it is included by connotation. The other is the downward resolution toward the sensible and observable object itself. In this process, being is not entirely left out (without this, no thinking remains). But being, or the idea of being, now ministers to the sensible object, especially to its measurable aspects. Its role here is that of an unknown or unobserved, which nevertheless assures the constancy of sensible determinations and measurements, and makes it possible to assign stable limits to the object perceived by the senses. This, in truth, is the method of resolving concepts in the experimental sciences. It is the resolution or analysis which I call empiriological or spatiotemporal, whereas to the other I give the name ontological (in the widest use of the word)."1
Accordingly, the manifestations of nature can be explained on two levels, one philosophical and the other scientific in the modern sense. This distinction leaves the physical sciences free to progress by methods of their own on their level of investigation, but it also admits of a philosophical consideration of nature in the manner of Aristotle. This would seem to be the correct approach, at least at first glance.
c) In point of fact, however, the respective limits of philosophical and scientific investigation are not so easy to determine as might at first appear. The philosopher of nature cannot altogether ignore the discoveries of science; but the scientist himself may not be justified in neglecting what the former has to say about such matters as finality, chance, space and time, and many others. It must furthermore be acknowledged that the aforesaid distinction between the philosophy and the science of nature is not found in so many words in Aristotle.2 Placing perhaps too much reliance on the deductive or a priori method of studying nature, Aristotle lumps his philosophy of nature with much that we should apportion to the scientist instead of the philosopher. Hence, within Aristotle's own physical doctrine one should distinguish between its philosophical and its scientific content. Its philosophical truth abides, as will appear in the sequel. The same cannot be said for what concerns the science of his day. This needs to be overhauled from the ground up.
Clearly, then, the task is not easy that awaits the author of a modern cosmology in the manner of Aristotle but without much of the Aristotelian matter. This author must perform a double feat in one. He will have to be constantly separating from the Aristotelian doctrine those portions that are scientifically outmoded, while holding on to those of permanent value. But this is only the half of it. On the foundation of his sifted Aristotle, he must build his superstructure, a theory of the universe that is solely philosophical. Moreover, to accomplish this task he will have to considerably enlarge the Aristotelian foundation, so as to take into account the mathematical techniques of the moderns.
Our scope is more modest. Here and there, to be sure, we have entered reservations to the Aristotelian doctrine. We have also, as occasion required, made note of contemporary theories. But our basic aim has been to give a true account of Aristotle's understanding of the physical world, and mainly of its philosophical content, the abiding feature of his study. As for its nonphilosophical admixture, or the ceaseless succession of ideas in what is now science proper, these questions are outside our principal theme; advertence to them is mostly by obiter dicta.