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A class is a special kind of Set invented to get around Russell's Paradox while retaining the arbitrary
criteria for membership which leads to difficulty for Sets. The members of classes are Sets,
but it is possible to have the class 
of ``all Sets which are not members of themselves'' without producing a
paradox (since is a proper class (and not a Set), it is not a candidate for membership in ).
See also Aggregate, Russell's Paradox, Set