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The Thomists (e.g., Feser, SM 3.1.1, 177-8), when establishing hylomorphism – the doctrine that a body is composed of prime matter (hayula) and form – favour, following Aristotle, a line of argument which proceeds from change.

The gist of their reasoning goes like this: change, understood as a transition from potency to act, presupposes an indeterminate but determinable element, called prime matter, as the bearer of the potency and a determining factor, called form, as the actuality. Change is real; therefore, prime matter and form exist. (For more detail, see here).

The Avicennians, on the other hand, though no doubt committed to hylomorphism, never, as far as I’m aware, make use of this argument. It is nowhere to be found in the works of the shaykh himself. I wonder why? Is it because he simply wasn’t aware of it? That’s hardly plausible, since he knew and read Aristotle’s Physics. Perhaps it’s because he didn’t consider it strictly an argument, but more an exposition? That’s plausible. Or, even if he did think it was an argument, maybe he didn’t think it was demonstrative? That seems more plausible. Assuming the latter to be the case, I want to register a criticism of the argument that he may have entertained.

So is the from change argument sound? Well, as Islamic philosophers would say, fihi nazar – for it may be legitimately questioned why the determinable element must be prime matter. Why, in other words, can’t it be just matter – an element which is, in itself, already determined to some extent but still further determinable, i.e., by the oncoming, determining form? Nothing about change understood as a transition from potency to act rules this out. Besides, this sort of thing is what precisely underlies accidental change, which suggests that it may also be the case in substantial change.

You might wonder: what would such an element be? Answer: body, understood as a substance extended in the three dimensions. So for example, when water ceases to be and air comes to be, matter as extension underlies the change. At the instant the ‘watery form’ vanishes, the ‘airy form’ informs the body of which the ‘water form’ was the form. And this body qua extension is in fact what’s common to them. In that way, it seems body can be the determinable element of the potency for the relevant actuality in a given substantial change.

You might worry though: ‘wouldn’t that make substantial change an accidental one’. Response: no, it wouldn’t, and for two reasons. First, an accidental change is a change in one of the accidental categories. But we agree that this here isn’t a case of that: one substance (i.e., water) corrupts and another is generated (i.e., air) – matter qua extended substance underlying, and hence persisting, throughout the change. And second, this claim (where C=body, A=water, and B=air):

C was A but not B, now C is B and not A

is misleading if it’s meant to capture the structure of a substantial change like the one above. For it construes C as real identifiable entity apart from A or B. But that’s not necessary; there is no ‘body’, only bodies (e.g., water, air, etc.).

If, then, body can be the determinable element, the argument from change fails to establish its conclusion; at most, it gets us to matter, but not prime matter.

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