 Subsequently, in Hale and Wright 2001 (p. 315), Wright expressed reservations over whether “x = x” counts as a sortal predicate eligible to be prefixed by “the number of x such that”. Now Wright was inquiring after what “is wanted for the exorcism of anti-zero” (p. 314, emphasis added). His considered answer is that a term of the form #xFx will denote a number only if the concept F is both sortal and not indefinitely  extensible.[39]  So Wright subsequently hoped to make actual what he had previously alleged to be impossible to imagine. The technical proposal must therefore be that Hume’s Principle is to be restricted to predicates that (express concepts that) are both sortal and not indefinitely extensible. But this, of course, raises the question whether there is an effective method for determining, of any given predicate F, whether F (expresses a concept that) is both sortal and not indefinitely extensible. In the absence of any such effective method, the theory will not have been axiomatized. 