https://plato.stanford.edu/entries/russell-paradox/
QT:{{”
John von Neumann’s (1925) untyped method for dealing with paradoxes, and with Russell’s paradox in particular, is simple and ingenious. Von Neumann introduces a distinction between membership and non-membership and, on this basis, draws a distinction between sets and classes. An object is a member (simpliciter) if it is a member of some class; and it is a non-member if it is not a member of any class.
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