The basis for our intuitive set theory is the Zermelo—Fraenkel set theory developedby Zermelo (Untersuchungen tiber die Grundlagen der Mengenlehre J) andstrengthened by Fraenkel (Zu den Grundlagen der Cantor—Zermeloschen Mengen-lehre). Their work rests on the researches of Cantor in the 1870’s which first putmathematics firmly on a set-theoretic base. Zermelo’s work, in particular, was adirect response to the Russell paradox.