  These unresolved problems of axiomatics led Hilbert to devote significant effort to work on logic in the following years. In 1917, Paul Bernays joined him as his assistant in Göttingen. In a series of courses from 1917–1921, Hilbert, with the assistance of Bernays and Behmann, made significant new contributions to formal logic. The course from 1917  (Hilbert, 1918b),  in particular, contains a sophisticated development of first-order logic, and forms the basis of Hilbert and Ackermann’s textbook Principles of Theoretical Logic  (1928)  (see  Ewald and Sieg 2013,  Sieg 1999,  and  Zach 1999, 2003).