 Also, with Gödel’s work around 1940 (and also with forcing in the 1960s) it became clear why the research of the 1920s and 30s had stagnated: the fundamental new independence results showed that the theorems established by Suslin (perfect set property for analytic sets), Sierpinski (\(\Sigma^{1}_{2}\) sets as unions of \(\aleph_{1}\) Borel sets) and a few others were the best possible results on the basis of axiom system ZFC. This is important philosophically: already an exploration of the world of sets definable from the open (or closed) sets by complement, countable union, and projection had sufficed to reach the limits of the ZFC system. Hence the need for new axioms, that Gödel emphasized after World War II [Gödel 1947].