50. General Principles. -- Nature and kinds of corporeal motion. Metaphysics studies motion in general; physics is concerned with corporeal motion and its kinds. These are four in number; appearance and disappearance of substantial compounds (genesis and phthora), qualitative change or alteration (alloiôsis), the quantitative change of growth and decay (auxêsis and phthisis), and finally local motion (phora) -- the motion par excellence, which the three former sorts presuppose. To the concept of local motion Aristotle attaches the study of the infinite, of time and of space.

The theory of matter and form belongs properly and primarily to physics, for it is the peripatetic interpretation of the evolution of the cosmos and of the incessant change that goes on in the world of sense. In opposition to the atomism of Democritus, which accounts for the visible diversities in corporeal things by different arrangements of the same identical elements, Aristotle contends that the facts of nature proclaim the existence of specific differences in corporeal things themselves and in their properties. Earthly substances are being continually transformed into one another; they combine with one another to form compounds specifically distinct from the components, and those compounds are themselves in turn resolved into their constituent elements. We must needs therefore recognise in corporeal substances a permanent substratum, primal or primary matter (hêprôtê hulê) identical throughout all stages of the process, and another principle peculiar to each one of these stages, the substantial form (eidos). The substantial form is so called because it is the first in order of all corporeal determining principles or influences, the one which fixes the substantiality and determines the kind or species of the thing; while the primal matter is the absolutely indeterminate substrate, incapable even of existing without the initial determination of the form. The succession of different forms in the same matter furnishes an explanation of the fundamental theory which Aristotle opposed to Plato, -- the theory of a real evolution taking place within the very entrails of corporeal things themselves. An actually constituted, existing corporeal substance is regarded as second or secondary matter with respect to the ulterior modifications or accidental forms it may receive.

The material world is therefore plunged in a whirlpool of Incessant change, as Heraclitus had already proclaimed, and yet none the less must we recognise, with Parmenides, a certain stability in its elements. To get an accurate insight into the mind of Aristotle regarding the process of cosmic change we must take careful cognisance of a twofold influence affecting the elements of the substantial compound, -- the rhythmic evolution of forms and the prevalence of purpose or finality.

Rhythmic Evolution of Forms. -- The theory is not fully and explicitly developed by Aristotle, but he has certainly the germs of a doctrine that was elaborately evolved and illustrated in the Middle Ages. In various passages in the Physics and Metaphysics Aristotle refers to a third principle, in explanation of the genetic process of nature, -- privation (sterêsis). By this he means the absence of a form demanded by the matter. This peculiar exigency on the part of the matter springs from a special tendency it has to divest itself, so to speak, of one form in order to assume another, when, under the influence of the surrounding natural agencies, the compound is in process of transformation. This gradual transition from form to form is regulated by the principle of the rhythmic evolution of substances. "You cannot make a silk purse out of a sow's ear."^{1} The plasticity of matter has therefore its laws and limits. This is merely the natura non facit saltus transported into philosophy.

The finality inherent in all being guides this process at every single step. Just as each individual step in the cosmic evolution tends to some new actualization of a potentiality of matter, so also is the sum-total of all these stages governed by a fixed purpose which Nature unswervingly and uninterruptedly pursues. Admirably equipped as he was -- for his time -- with a fund of scientific observations, Aristotle followed out in detail the applications of teleology to all the facts of Nature. There is in ancient philosophy no more eloquent advocate of final causes than he.

And what is this final term, this end towards which all Nature tends? It is that which is the most perfect, the Purely Actual (45), Aristotle replies. But this gives rise to many questions for which he has no satisfactory answer: Does this impulse of the creature towards God imply some vague sort of knowledge of its end in every existing being? Does the unity of order, which combines the various substances of Nature and harmonizes their activities, involve a sort of organic unity in nature (phusis), a "world-soul" endowed with some faint perception of its evolutions and of their term? And if so, how are we to reconcile such unity with the individuality of the beings included in it (43), or to recognize the distinction between the organic and the inorganic worlds? (52).