 The German Marburg School of Hermann Cohen, Paul Natorp, and Ernst Cassirer was especially important in the emergence of modern philosophy of science in the form of the logical positivism and logical empiricism. Friedman (2001 and elsewhere) has explored its influence on young Reichenbach’s attempt to interpret the significance of the new relativity theory. Rudolf Carnap had been influenced by Ernst Cassirer, among others. (See the entries on logical empiricism, Reichenbach, Carnap, Cohen, Natorp, and Cassirer.) Cassirer’s central theme was the fundamental epistemological implications of the replacement of Aristotle’s subject-predicate logic and substance ontology by the new relational logic of the modern, mathematical-functional approach to nature. This could not be a priori in Kant’s original sense, since the emergence of non-Euclidean geometry had shown that there are alternative organizing principles. But the very fact that we still needed organizing structures that are constitutive or definitive of the cognitive enterprise in question meant that Kant was still basically correct. Like Moritz Schlick, the first leader of the Vienna Circle, Reichenbach of the Berlin school parlayed his engagement with relativity theory and non-Euclidean geometries into a conception of the “relativized a priori.” All of this by around 1920, with the younger Carnap’s views developing during the 1920s and ‘30s. In the USA, meanwhile, C. I. Lewis (1929) was defending his “pragmatic a priori.”