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 di