 The father of set theory, Cantor, had noticed similar difficulties already in 1895 (as witnessed by Bernstein and by letters to Hilbert and Dedekind). Indeed, in a second letter to Dedekind of August 31, 1899 Cantor pointed out another problem, involving the notion of the cardinal number and implying that one cannot consistently think of the “the set of all conceivable sets”, say \(M\). Were \(M\) a genuine set, then it would possess a cardinal number \(m\), which would be the maximum cardinal number. But one could also consider the set \(\wp(M)\) of all subsets of \(M\), and by Cantor’s theorem the cardinality of \(\wp(M)\) ought to be strictly bigger than the purported maximum \(m\): contradiction.