As we have seen above, between 1907 and 1911 the proto-circle of what would later emerge as the Vienna Circle was formed with Hans Hahn, Philipp Frank, Otto Neurath, and also Richard von Mises. Essential elements of Logical Empiricism were already anticipated in that discussion group, but World War 1 represented a radical caesura within this intellectual development. It could not, however, permanently stop the efforts to bring about a renewal and a "turn in philosophy." The return of Hans Hahn to Vienna as professor for mathematics in the summer semester of 1921 signified the launching of scientific philosophy in terms of content and organization.