# [FOM] independence of extensionality

Thomas Forster T.Forster at dpmms.cam.ac.uk
Tue Apr 26 15:57:38 EDT 2005

Who was it who first proved the axiom of extensionality from the other
axioms of ZF?  I'm thinking in particular of tricks like: all sets are
empty (in the new sense)  unless they are singletons and $x \in \{y\}$
(in the new sense) if $x \in y$ (in the old).   Was it Fraenkel and
Mostowski? And can anyone supply a reference..?

Thomas

URL: www.dpmms.cam.ac.uk/~tf  tel: 01223-337981 and 020-7882-3659