I raised the question here whether by 'primary substance' in the Categories Aristotle means simply particular things (as the common wisdom goes), or whether he means something like an instance of a nature. There was a simple and so far unanswered argument for the latter: the categories are highest kinds of existences; thus, substances are distinct existing things from accidents; thus, the primary substance of a thing must be something distinct from any of its accidents; and therefore the substance of a thing is distinct from the thing, which would include its accidents.
In subsequent posts I pointed out that, when a primary substance is thus understood as 'an instance of a nature', then the Categories becomes aligned with the Metaphysics in important ways.
First, it becomes natural to speak of the substance of a thing, since the substance is not the thing. This idiom is not a strange, unaccountable innovation of Z3, but simply what has been presumed all along. (Yes, there remains an everyday sense in which people want to say that things are substances. But the Categories shows that this way of speaking needs justification: once we clarify what the primary substance is, this usage appears imprecise and derivative.)That is, the entire project of Z comes out of the conception of primary substance as an instance of a nature: the investigation of hypokeimenon; essence; universal; genus.
Second, the 'matter' of a primary substance (the stuff of which it is composed) must be conceived as in some important way 'indeterminate': the primary substance being distinct from definite accidents of quantity, place, and position, any 'stuff'' it may have will be so as well. And yet Aristotle investigates indeterminate matter in the Metaphysics.
Third, we might wonder whether an indeterminate thing could still be a subject, and in what way.
Fourth, since it appears problematic how something indeterminate may still be matter, the Categories view of primary substance invites investigation into to what extent the matter of a thing should enter into its essence or definition.
Fifth, by the same token, to the extent that it appears problematic that matter does enter into the essence and definition of a substance, to that extent it will appear likely that the substance is somehow constituted by universals or kinds.
But I wonder, still, whether there is good textual evidence in the Categories to support the 'unanswered' argument given above. As I said, I think I have found some, and I'll give it in a subsequent post.