It was in 1922 that Moritz Schlick — a German physicist-cum-philosopher of aristocratic manners and conservative opinions — arrived in Vienna¹. He had been invited to take up chair of philosophy with special reference to the history and theory of the inductive sciences' that had been created for another physicist-cum-philosopher, Ernst Mach, in 1895. Mach himself had previously served for almost thirty years as professor of experimental physics in Prague, and it will be important in what follows to remember that Prague was still to some extent a German city and a centre of intellectual activity almost no less important than Vienna herself. The lines of communication between the two former Imperial-Royal capital cities² were still strong, and the same figures were often, at different times, prominent in each.