Solution
= Solution
\b[No such set exists.] If $R$ had precisely the <sets> $x$ satisfying $x\notin x$ as its elements, applying that membership criterion to $R$ itself would give
$$
R\in R\quad\Longleftrightarrow\quad R\notin R,
$$
a <contradiction>. This is <Russell's paradox>. It rules out this unrestricted universal collection; a <set> whose elements happen not to contain themselves is perfectly possible, for example the <empty set>.