“…Theorems 4.2 and 4.3 mean that Or S is the only S-ordinal which does not belong to S. Now we consider the axiom of replacement. It is well known [3,6] that A. Fraenkel and T. Skolem had independently proposed adjoining replacement axiom to establish that p(p(ω)), ...} be a set since, as they pointed out, Zermelo's axioms cannot establish this. However, even E ∅ cannot be proved to be a set from Zermelo's axioms.…”