 However, one could raise at least two objections against the theory on different grounds. First of all, Zermelo’s approach is actually highly impredicative and impredicativity was regarded as indispensable by him (else one would be forced to reject standard mathematics, e.g., Zermelo believed this was the case even with the Cauchy-Weierstrass proof of the fundamental theorem of algebra). But impredicativity makes the construction of a model or of an interpretation more difficult and less evident. The second point is that Zermelo believed that the paradox of finite denotation (mentioned by Hessenberg) and Richard’s are blocked in set theory, as the separation axiom ought to yield clear criteria for defining sets. But this is not the case, because Zermelo’s notion of definite property (definite Eigenschaft) is given informally and is ultimately vague.