 Unlike Burali-Forti’s paradox, Russell’s paradox does not involve either ordinals or cardinals, relying instead only on the primitive notions of set and set inclusion. Zermelo noticed a similar contradiction sometime between 1897 and 1902, possibly anticipating Russell by some years (Ebbinghaus and Peckhaus 2007, 43–48; Tappenden 2013, 336), although Kanamori concludes that the discovery could easily have been as late as 1902 (Kanamori 2009, 411). As Linsky points out, Zermelo’s argument, while similar to Russell’s, is best understood as one of a cluster of arguments by Zermelo, Schröder and Cantor that “indeed anticipated” the mathematical argument Russell developed but that turned out to be different in small but significant ways from Russell’s argument (Linsky 2013, 11). In any case, the arguments were thought to be of minor importance until it was realized how detrimental they were to Gottlob Frege’s  foundations for arithmetic.