Russell's Paradoxes

Book: The Man from the Future: The Visionary Life of John von Neumann

Let RR be the set of all sets that are not members of themselves. If RR is not a member of itself, then its definition entails that it is a member of itself; if it is a member of itself, then it is not a member of itself.

R={xx∉x}R = \{x | x \not\in x\}

Then,

RR    R∉RR \in R \iff R \not\in R