A Refined Begging of the Question

Yesterday I proposed a refinement in Devereux's interpretation of Z 3, a correction that he ought to accept, given his approach to that passage. I wonder if you noticed to what extent this refinement, if accepted, weakens his claim to be giving a correct interpretation.

I argued that, on Devereux's interpretation, Aristotle wouldn't need to use the concrete substance as a kind of link or intermediate term in an argument that aimed to show that everything besides the concrete substance was predicated of the underlying substratum, and that Devereux's interpretation, then, really amounts to:

1. Unless Aristotle is supposing that (what he calls) 'substance' and 'this' (in the above passage) is predicated of the ultimate substratum, then Aristotle fails to establish that every existent besides the ultimate substratum is not a substance.
2. (What Aristotle calls) 'substance' and 'this' (in the above passage) is a concrete substance.
3. Thus, Aristotle is supposing that a concrete substance is predicated of the ultimate substratum.

But then this argument has Aristotle committing petitio principii. The only claim Aristotle really needs, to show that the ultimate substratum is the only substance, is that concrete substances are predicated of something else. That's precisely what, according to Devereux's interpretation, Aristotle is now 'presupposing'.

Aristotle does not assert in that passage, on its own, that concrete substances are predicated of the substratum. He gives no argument for this. And yet this would be precisely what is at issue (and Aristotle would have been well aware that this was at issue).

On Devereux's interpretation, Aristotle fails to state or defend the only thing that he needs to state or defend.