a) Basic supposition. - Aristotle assumes the fact of change and motion. "We must take for granted," he says, "that things of nature, either all or some, are in motion. This, as a matter of fact, is clearly evident by induction." 3 The declared foundation on which Aristotle rests his demonstration of three principles, and indeed his entire physics, is the reality of change, a reality of immediate experience. To the Eleatic doctrine of monism and unchangeableness he opposes first and foremost the incontrovertible evidence of experience. Generation and other changes are simply facts, stark and unmistakable. Who was illiterate becomes literate. What was black becomes white. Learning and teaching, whitening and coloring are real; we can see them, watch them, do them. Mere things like these are enough to override the Parmenidean denial of change, which, to say the least, is fraught with inconsistencies. By contrast the physics of Aristotle boldly declares itself a doctrine of change and of changeable (mobile) being.
With the acceptance of change or movement goes the acceptance of multiplicity. Being that changes undergoes successive multiplicity and has therefore to be made of more than one element or principle. For that matter, multiplicity, like change itself, is a fact of immediate ex- perience. Accordingly, Aristotle's world is both multiple and changeable. But its truest characteristic is change rather than multiplicity, because only natural being is susceptible of movement proper, which means change, whereas multiplicity is not the sole property of natural being but is found as well in immaterial being, in the world of spiritual substances.
b) The principles are contraries.4 - Having affirmed his position on the question of change and motion, Aristotle proceeds to unfold his doctrine of principles. The first thing he does is to show that they must be contraries. All the early Physicists, he believes, were in agreement on this. His own reasoning in the matter begins, characteristically, with items of obvious experience, like a colored body changing to white.
What are the basic requirements for a colored body changing to white? Common experience shows that this process embraces two terms or footings: the term acquired (terminus ad quem), which is whiteness, and the starting point (terminus a quo), which is the original color, or better, the nonpossession of whiteness. In other words, there is a passage from nonwhite to white. Speaking generally, we may designate the ultimate term of this change as form, and its point of departure as privation. Accordingly, we may say that every change transpires between two opposite terms: one the absence or privation of a given physical determination, the other the real acquisition of this determination, or form. Privation and form, these, in consequence, are two primary principles of change. In the next heading we shall see that there is still a third.
A close reading of Aristotle's chapter 5, Book I, of which the preceding paragraph is a summary, will show where his preoccupation lay and why he turned to contraries for the explanation of change. What Aristotle sought was two terms or points of reference that would be independent of each other and first in their order - contraries (by the ancient physics) plainly satisfied this condition. But there had also to be some community, some common ground, between the two terms, since white, for example, does not come from whatsoever thing or term, but from nonwhite (which belongs to the same genus color). Briefly, what Aristotle wanted to get across was that change is insoluble except on principles that are opposites and independent of each other, yet members of the same genus. Contraries met the test.
c) Necessity of a third term.5 - Contraries alone are not enough to account for change. Change, after all, means to become what one was not, and this implies that in some respect one remains what one was. There is a selfsameness that outlives every change, a floor or base that stretches from one term of change to another and successively lodges both. When one term is lost, another is gained. But besides what is lost and gained there has to be something that loses the one and gains the other. If we think of change as a complete break between its terms, then its whole meaning is destroyed and change is indeed impossible.6
Contraries, however, cannot supply this continuity from one term to another, as they can neither act on each other nor be born of each other. Besides, substance has no contrary. If, then, contraries were the only principles of change, substance would have to be generated of something that is not substance, a manifest impossibility. In short, every contrariety must rest on something that is without contrariety. Thus, a third term is needed, a subject, also called matter. This provides the foundation for the process of change as well as for the terms that bound its course. Given a subject, change becomes intelligible. A subject in privation to a form acquires that form; a nonwhite body becomes a white body. This is the meaning of change.
In proving that the principles are contraries plus a subject, Aristotle also takes pains to show that more than three are not necessary and, most of all, that the principles cannot be infinite or unlimited in number. In a word, every change in the physical world requires:
d) Solution of the Eleatic difficulty.7 Of all his predecessors none taught a doctrine more radically opposed to Aristotle's theory of change and becoming than Parmenides and the Eleatics generally. For this reason Aristotle, having stated his own position, returns now to the refutation of the Eleatics. They, in brief, asserted that becoming is impossible because being cannot come from being (this already is), and it cannot come from non-being, which, they said, is utter nothingness.
Aristotle's answer is that generation or becoming springs both from a kind of being, that of the subject, and from a kind of nonbeing, that of privation. Thus, the Eleatic dilemma is not airtight; it admits of escape. Aristotle proposes, without elaborating, yet another answer to the Eleatic difficulty, based on one of the most important distinctions in his metaphysics, namely, the distinction of act and potency.
Becoming, he notes, is a transition from one mode of being to another, from being in potency to being in act. So, in the earlier example of whitening, what is white in potency becomes white in act. Change, then, is possible because between being and utter nothingness there is an intermediate state, which is potential being or being in potency.
e) Conclusion. - As we have seen, change is the distinctive mark of physical being. Three principles - matter or subject, privation, and form - are necessary to account for change. Considered in its essentials, Aristotle's analysis appears beyond questioning, and the scientific advance that would compel a basic revision has not yet been made, nor can one conceive that it will be made in the future. In Aristotelian thought, moreover, the notions of matter, form, and privation are corroborated by their relevancy to other philosophical problems, especially to the individuation of material substances, and to the correlative problem, their multiplication. Additional proof is also seen in the fact that the positive or intrinsic principles of bodies, matter and form, serve admirably to account for the opposition between certain properties of bodies, both in the order of quantity and quality. But the basic evidence remains the simple fact of change.
Accustomed as he is to studying the physical world much differently, the modern scientist, more often than not, confesses to a certain discomfiture in face of Aristotle's conceptions. What he needs to remember is that Aristotle's analysis of nature is in the pattern of his predecessors, and should be understood in the light of their physical doctrines rather than on the basis of present-day physical theories. In particular, the role he assigns to contraries cannot be truly deciphered except against the thought of his predecessors. These men, for all their obviousness, were not without basic insight. To them the world was a vast battleground in which contrary elements, like warm and cold, dry and wet, light and darkness, were pitted against each other; and it was almost inevitable that they should
go a step further and declare opposites, or contraries, the ultimate principles of things and their transformations. The point is that the speculations of an Anaximander, a Heraclitus, or an Empedocles were not lost on Aristotle. If we bear this in mind, his doctrine of contraries ceases to be an oddity and assumes a perfectly normal cast.8