 The next large step was taken by David Hilbert in his lecture course Prinzipien der Mathematik, delivered in Göttingen in the winter semester of 1917/18. Hilbert had lectured and published on foundational topics in the years 1899–1905; in the intervening time, as he concentrated on other matters, the publications had ceased, though the extensive classroom lecturing continued. He kept up with current developments, and in particular was informed about the logical work of Whitehead and Russell, largely through his student Heinrich Behmann. In September, 1917, he delivered his programmatic lecture “Axiomatisches Denken” in Zürich calling for an axiomatic treatment of logic along the lines he had earlier explored in his axiomatization of geometry, and explicitly proposing metalogical investigations: