. Most classes are not members of themselves (the class of cats is not a cat), but some classes are members of themselves (the class of classes is a class). Is the class of all classes that are not members of themselves a member of itself? If yes, no....

Part of the foundation of mathematics, Russell's paradox (also known as Russell's antinomy), discovered by Bertrand Russell in 1901, showed that the naive set theory of Frege leads to a contradiction. It might be assumed that, for any formal criterion,...