  After Gödel had made public the incompleteness of arithmetic in September 1930, von Neumann found that the consistency of arithmetic would be among the Gödelian unprovable propositions. Alas, Gödel had made the same discovery so von Neumann never published his proof. He did, however, conjecture in correspondence with Gödel the unprovability of the consistency of arithmetic and therefore of mathematics as a whole, in some absolute sense. Von Neumann was the key character in the reception of Gödel's results: He interrupted his lectures on Hilbert's proof theory in Berlin in the fall of 1930 to explain the new discoveries. These events created an enormous excitement among the mathematicians, as witnessed by Carl Hempel's testimony: