 Russell appears to have discovered his paradox in the late spring of 1901, while working on his Principles of Mathematics (1903). Exactly when the discovery took place is not clear. Russell initially states that he came across the paradox “in June 1901” (1944, 13). Later he reports that the discovery took place “in the spring of 1901” (1959, 75). Still later he reports that he came across the paradox, not in June, but in May of that year (1969, 221). Cesare Burali-Forti, an assistant to Giuseppe Peano, had discovered a similar antinomy in 1897 when he noticed that since the set of ordinals is well-ordered, it too must have an ordinal. However, this ordinal must be both an element of the set of all ordinals and yet greater than every such element. 