 The publication of Volume II of Frege’s Grundgesetze (1903), and above all Russell’s work The Principles of Mathematics (1903), made the mathematical community fully aware of the existence of the set-theoretic paradoxes, of their impact and importance. There is evidence that, up to then, even Hilbert and Zermelo had not fully appreciated the damage. Notice that the Russell-Zermelo paradox operates with very basic notions—negation and set membership—concepts that had widely been regarded as purely logical. The “set” \(R = \{x: x \not\in x\}\) exists according to the principle of comprehension (which allows any open sentence to determine a class), but if so, \(R \in R \textit{ iff } R \not\in R\). It is a direct contradiction to the principle favoured by Frege and Russell.