  However, before arriving in Vienna, Schlick engaged the neo-Kantians,  Hans Reichenbach and  Ernst Cassirer, who had published their own philosophical understandings of the new physics of Relativity. In his Relativity Theory and Apriori Knowledge of 1920, Reichenbach had argued for a modified conception of Kant’s synthetic apriori, which challenged Schlick’s thought in a decisive way (Reichenbach 1920). At Einstein’s request, Schlick wrote to Reichenbach in the Fall of 1920, hoping to air the differences between them (Schlick 1920; Einstein 1920). In his own essays, Schlick had challenged Kantian apriorism principally by identifying the presuppositions of the new physics as conventions, in Poincaré’s sense. Since the presuppositions of Relativity were alien to classical physics, they were nothing like the self-evident, eternal verities that comprised the Kantian apriori. But while Schlick rejected Kant’s treatment of the apriori altogether, Reichenbach claimed to have preserved its most important element, its constitutive function (Reichenbach 1920, Ch. V; Friedman 1999, 59–70; Oberdan 2009). For Kant had attributed the apriori the far more philosophically significant function of constituting the object of experience or knowledge. Indeed, such principles are the general laws for ordering experience to produce knowledge. Since all empirical knowledge presupposes these ordering principles, they can never conflict with experience and are, in this sense, necessarily true.  (Reichenbach 1920, pp. 55–56) Schlick first wrote Reichenbach in the Fall of 1920, acknowledging that he regarded the assumption of constitutive principles as self-evident; indeed, he feared that the matter was so obvious that he might not have discussed it sufficiently in his General Theory of Knowledge (Schlick 1920a, p. 1; Oberdan 1994, pp. 109–110). Nonetheless, it is precisely the principles which Reichenbach himself had identified as synthetic apriori that constitute an observation or measurement of an experience. Yet, Schlick confessed, he was unable to discover any characteristics of these alleged synthetic apriori principles that genuinely distinguish them from conventions. Of course, it then follows that the precepts Reichenbach called “synthetic apriori” were just what Schlick had identified as “conventions” and the differences between them were, at most, terminological (Schlick 1920b, 2). But this apparently terminological difference, Schlick thought, masked a far deeper difference separating them since, on Reichenbach’s understanding, the apriori constitutes the objects of experience and knowledge whereas, on Schlick’s, conventions only constitute concepts, which may be applied to experiences and objects, but do not constitute them. Thus Schlick insisted on distinguishing his own realist epistemology from Reichenbach’s modified Kantianism, because of the latter’s implicit anti-realism.