 In assessing this argument, a lot seems to depend on how extensive an explanatory role can be assigned to hypothetical necessity (cf. Physics ii 9). All human beings have a tendency to fall, necessarily, at least in a world with laws of physics like ours. However, it is not so clear whether this characteristic sort of change is one which must be explained by the form or essence of a human being, as opposed to his matter. After all, there are lots of other sorts of thing, both living and inanimate, which share this particular characteristic. Supposing there was a characteristic sort of change peculiar to all and only human beings, even then it is not obvious (a) that this fact has to be explained by the essence of a human being, and (b) that its explanation will require the essence to be matter-involving. To be sure, we would like some explanation of why this sort of change is peculiar to this sort of creature, but it might simply be a fact about the world that anything with an essence of this sort has to change in this sort of way, without that change being something that is specified within the essence itself.