 In that same year, Stanislaw Ulam was led by considerations coming out of analysis (measure theory) to a concept that was to become central: measurable cardinals. It turned out that such cardinals, defined by a measure-theoretic property, had to be (strongly) inaccessible. Indeed, many years later it would be established (by Hanf, working upon Tarski’s earlier work) that the first inaccessible cardinal is not measurable, showing that these new cardinals were even more “exorbitant”. As one can see, the Polish school led by Sierpiński had a very central role in the development of set theory between the Wars. Measurable cardinals came to special prominence in the late 1960s when it became clear that the existence of a measurable cardinal contradicts Gödel’s axiom of constructibility (\(V = L\) in the class notation). This again vindicated Gödel’s convictions, expressed in what is sometimes called “Gödel’s program” for new axioms.