#OTD in 1902.
Bertrand Russell writes to Gottlob Frege about the mathematical problem to become known as Russell's paradox.
Russell's paradox shows that every set theory that contains an unrestricted comprehension principle leads to contradictions. Several responses and resolutions to Russell's Paradox have been developed such as: Zermelo-Fraenkel Set Theory ; Type Theory proposed by Russell himself; Von Neumann–Bernays–Gödel Set Theory, among others.