[T]here are different senses of ‘coming to be.’ In some cases we do not use the expression ‘come to be,’ but ‘come to be so-and-so.’ Only substances are said to ‘come to be’ in the unqualified sense.
Now in all cases other than substance it is plain that there must be some subject, namely, that which becomes. For we know that when a thing comes to be of such a quantity or quality or in such a relation, time, or place, a subject is always presupposed, since substance alone is not predicated of another subject, but everything else of substance.
But that substances too, and anything else that can be said ‘to be’ without qualification, come to be from some substratum, will appear on examination. For we find in every case something that underlies from which proceeds that which comes to be; for instance, animals and plants from seed …
The underlying nature is an object of scientific knowledge, by an analogy. For as the bronze is to the statue, the wood to the bed, or the matter and the formless before receiving form to any thing which has form, so is the underlying nature to substance, i.e., the ‘this’ or existent.
Aristotle has been arguing that in any case of change, something must persist—that is, there must be something that undergoes the change. Why is he so sure of this? How would you describe a case where a change happens, but there is nothing numericallyidentical throughout it?
Assuming this principle—in any change, there must be something that endures through the change—is sound, we need to look attwo very different kinds of case. Take the case of not-bald/bald. What is the ‘underlying substratum’ in this sort of case?
But now consider a substance itself coming to be (i.e., instead of coming-to-be-F, consider coming-to-be period.) There must be asubstratum here as well; but it cannot be a substance (since this is not a case of some substance taking on a new property, but cominginto existence in the first place.) This is prime matter, matter lacking all form.
- We never experience prime matter; how, then, does Aristotle think we can come to know it? (See the previous paragraph of Aristotle’s text)