 Second-order ZFC, \(\ZFC^2\), is simply the received first order ZFC with the Separation Schema replaced by the above single Separation Axiom, and the Replacement Schema replaced by the above single Replacement Axiom. Accordingly, \(\ZFC^2\) is a finite axiom system. Zermelo proved that the models of \(\ZFC^2\) are, up to isomorphism, of the form \((V_\kappa,\in)\), where \(\kappa\) is (strongly) inaccessible \(&gt;\omega\).