18958 | In type theory, 'x ∈ y' is well defined only if x and y are of the appropriate type [Putnam] |
18107 | A 'proper class' cannot be a member of anything [Bostock] |
13651 | A set is 'transitive' if contains every member of each of its members [Shapiro] |
15921 | Collections of things can't be too big, but collections by a rule seem unlimited in size [Lavine] |