From: utzoo!decvax!harpo!ihnp4!ihuxr!lew Newsgroups: net.math Title: Look Ma! I'm comprehending! Article-I.D.: ihuxr.322 Posted: Mon Feb 7 21:27:46 1983 Received: Wed Feb 9 05:34:27 1983 Reply-To: lew@ihuxr.UUCP (Lew Mammel, Jr.) Thanks to Yigal Arens and Jerry Leichter for their rapid and informative responses. I was confusing the substitution of y into a formula containing x as a free variable, with a formula containing y as a free variable. Jerry is right about the Monk book, he defines a set as "a class small enough to be a member of some other class." He defines a universal class, not a universal set. I think it is interesting that the ZF formulation still lets us define y={x: xy