diagonal argument. According to Hilbert, it even initiated Ernst Zermelo's version of Russell's paradox.

Informal presentation

Most sets commonly encountered are not members of themselves. Call a set "normal" if it is not a member of itself, and "abnormal" if it is a member of itself. Clearly every set must be either normal or abnormal. For example, consider the set of all squares in a plane. This set is not

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