After the axiomatization lectures of 1905, Hilbert lectured several times again on physical issues: courses on mechanics (WS 1905–06) and continuum mechanics (SS 1906; WS 1906–07), differential equations of mechanics (WS 1907–08), and another advanced seminar on electrodynamics together with Minkowski (SS 1907). Thereafter, he taught no additional courses on physics until 1910. In some sense this may be considered a period of lesser creativity in general for Hilbert, who suffered at the time of some kind of nervous breakdown.² We should not conclude, however, that over these three years physical issues disappeared from his horizon of interests. On the contrary, these were years of great excitement in Göttingen around Minkowski's ground-breaking contributions to electrodynamics and one may safely assume, even without direct evidence, that Hilbert was closely following these developments and that he continuously interacted with his good friend on these topics. Indeed, a close inspection of the contents of Minkowski's ideas reveals a substantial, direct connection with Hilbert's program for the axiomatization of physics, as it has been described so far. Minkowski's work can be seen to a large extent as a particular implementation of this program, whereby the specific, structural role of a new principle recently adopted in various physical theories the principle of relativity was thoroughly investigated for the first time.