a) We have spoken of the number of principles required for change. So far the discussion has dealt with change generally, and in Book I of the Physics Aristotle does not carry it further. The specific kinds of change and their severally specific principles are investigated in De Generatione et Corruptione .9 "We have to examine," Aristotle there says, "the whole question of absolute generation and absolute corruption, to see whether these changes do or do not occur, and if they do, to determine the conditions thereof; we must also consider the other kinds of change, such as growth and 'alteration.' " 10
The "examination" which Aristotle pursues in De Generatione leads him to the conclusion that there are two basic kinds of generation: absolute (or substantial) generation, which means a radical transformation of one thing into another; and relative (or accidental) generation, in which the subject or substrate remains essentially what it was but undergoes accidental or nonessential modification.
b) Aristotle's principal concern was with substantial generation; this, above all, had to be affirmed and defended, since it had been imperiled or impugned by two separate schools of thought. One was the school that believed all things are made of the same ultimate element; the other supposed several specifically distinct elements. For the proponents of the first view - Thales, Anaximander, Anaximenes - all change came to mere accidental modification of a primordial substance, water for one, air for another, and whatever element seemed likely to still others. The second group - the Atomists, also Empedocles and Anaxagoras - allowed that substances undergo a degree of innovation, but the change is no more than association or dissociation of preexistent elements, each retaining its separate and distinct nature. Either way, be the ultimate element one or many, change results in new aggregation, but never in new substance.
Here lay a challenge that Aristotle could not ignore. Not all change or generation is accidental. There is, he insisted, absolute generation, by which the pre-existent substance ceases to be and a completely new substance comes to be. Hence, the underlying principle of the new substance cannot be a substratum having its own determinate nature, or a plurality of elements already invested with specific determination; for then the new substance would not be one specific nature but a composite of two or more. No, the underlying principle can only be a subject or matter that is utterly undetermined. Such matter is necessary because, as we have seen, every generation or change requires a subject, but in absolute generation the subject cannot be a substance, since this is precisely what changes. The subject must therefore be a principle without any positive determination whatever, a principle to which we give the name "prime matter."
c) One difficulty on this score that troubles moderns does not seem to have worried Aristotle, namely, the practical recognition of substantial changes. For Aristotle, such changes are obvious, typical examples being the birth and death of a living thing and the transmutation, as Aristotle thought, of the basic elements into each other. For example, water in evaporating was thought to become air; and air, in heating, to become fire. Today, we should hardly accept these "transmutations" as convincing proof of substantial change. Nor do we share Aristotle's or his predecessors' certainty of having found the exact number of irreducible, substantial elements. And as for knowing whether this or that variation in the physical appearance of a thing denotes a substantial change, this is usually most difficult if not impossible.
Chemical changes may be more decisive, especially when they leave a marked difference of behavior in their wake. Here a substantial change may well be indicated. But physical and chemical changes aside, birth and death afford unequivocal instances of substantial generation. Dead matter leaps to life, as in nutrition or fertilization and conception, and living matter falls in death. These are facts. There is every reason to consider them substantial changes, and no logical ground to think them otherwise. In birth and death a new substance succeeds the old. To deny this would seem to stultify both thought and reality. In summary, the physical world registers two basic kinds of change. It manifests those superficial modifications called accidental changes and commonly seen or detected without difficulty. But it also shows changes that are, in the truest sense, substantial generations and corruptions, issuing in corporeal substances that are completely new.11