  The publication of Russell and Whitehead’s  Principia Mathematica provided the required logical basis for a renewed attack on foundational issues. Beginning in 1914, Hilbert’s student Heinrich Behmann and others studied the system of Principia (see  Mancosu 1999  on Behmann’s role in Hilbert’s school). Hilbert himself returned to work on foundational issues in 1917. In September 1917, he delivered an address to the Swiss Mathematical Society entitled “Axiomatic Thought”  (1918a).  It is his first published contribution to mathematical foundations since 1905. In it, he again emphasizes the requirement of consistency proofs for axiomatic systems: “The chief requirement of the theory of axioms must go farther [than merely avoiding known paradoxes], namely, to show that within every field of knowledge contradictions based on the underlying axiom-system are absolutely impossible.” He poses the proof of the consistency of arithmetic (and of set theory) again as the main open problems. In both these cases, there seems to be nothing more fundamental available to which the consistency could be reduced other than logic itself. And Hilbert then thought that the problem had essentially been solved by Russell’s work in Principia. Nevertheless, other fundamental problems of axiomatics remained unsolved, including the problem of the “decidability of every mathematical question,” which also traces back to Hilbert’s 1900 address.