A follow up point on De Int 16b19-25.
In that passage, the parenthetical remark seems especially difficult to explain, viz. "nor if you say simply 'that which is'" (ou)d ' e)a\n to\ o)\n ei)/ph|j au)to\ kaq' e(auto\ yilo/n). Ackrill's suggestion is clearly inadequate:
The logical point involved seems to be quite different from the main point of the passage, though naturally connected with it. Roughly: someone might suggest that if 'is' does not make a statement because it does not say what is, then surely 'what is' ('that which is') does make a statement--surely 'what is' does say what is. Aristotle is, of course, right to reject this suggestion, but the reason has nothing to do with the verb 'to be' in particular; no substantival expression can state that something is the case.But this construal is implausible in the context. Aristotle begins the passage by pointing out that verbs just on their own function as do substantivals. He insists that even 'is' (this would be the best contender for a counterexample) is not an exception. It would hardly then be to the point for someone to bring forward, now, a substantival expression (to\ o)\n) used just on its own. Clearly such an expression will function as a name; it could do nothing to show that verbs used in isolation function other than as names.
Yet the parenthetical remark does make sense, if we take the passage as an implicit criticism of Parmenides--with Aristotle presuming that 'that which is' (to\ o)\n) is naturally taken as the subject of Parmenides' 'is'.
The criticism goes as follows. Parmenides gives as his two ways, 'is' and 'is not'. But these are, so far, simply names, not premises or claims. We take Parmenides to be saying something, only because we supply a subject, 'that which is'. To see this, consider that 'is' no more says something about the world than 'that which is' on its own. And yet (understand) if we were to combine the two expressions, 'That which is, is', we would have a tautology which leads to no interesting conclusions beyond itself.