linguistic:paraphrasing
Russell's paradox

The resolution of Russell's paradox is trivial: x ∉ x {\displaystyle x\not \in x} is not a stratified formula, so the existence of { x ∣ x ∉ x } {\displaystyle \{x\mid x\not \in x\}} is not asserted by any instance of Comprehension. Quine said that he constructed NF with this paradox uppermost in mind.