 The most influential of all neo-Kantian interpretations of general relativity was Ernst Cassirer’s Zur Einsteinschen Relativitätstheorie (1921). Cassirer regarded the theory as a crucial test for Erkenntniskritik, the preferred term for the epistemology of the physical sciences of Marburg’s transcendental idealism. The question, posed right at the beginning, is whether the Transcendental Aesthetic offered a foundation “broad enough and strong enough” to bear the general theory of relativity. Recognizing the theory’s principal epistemological significance to lie in the requirement of general covariance (“that the general laws of nature are not changed in form by arbitrary changes of the space-time variables”), Cassirer directed his attention to Einstein’s remarks, cited in  §2  above, that general covariance “takes away from space and time the last remnant of physical objectivity”. Cassirer correctly construed the gist of this passage to mean that in the general theory of relativity, space and time coordinates have no further importance than to be mere labels of events (“coincidences”), independent variables of the mathematical (field) functions characterizing physical state magnitudes. In particular, the requirement of general covariance had significantly improved upon Kant in bringing out far more clearly the exclusively methodological role of these conditions in empirical cognition, a role Kant misleadingly assigned to pure intuition. Not only has it has shown that space and time are not “things”, it has also clarified that they are “ideal principles of order” applying to the objects of the physical world as a necessary condition of their possible experience. According to Cassirer, Kant’s intention with regard to pure intuition was simply to express the methodological presupposition that certain “intellectual forms” (Denkformen), among which are the purely ideal concepts of coexistence and succession, enter into all physical knowledge. According to the development of physics since the 17th century already chronicled in Substanzbegriff und Funktionsbegriff, these forms have progressively lost their “fortuitous” (zufälligen) anthropomorphic features, and more and more take on the character of “systematic forms of unity”. From this vantage point, general covariance is but the most recent refinement of the methodological principle of “unity of determination” governing the constitution of objects of physical knowledge, notable in physics by the transition from concepts of substance into functional and relational concepts. In accord with central tenets of the Marburg Kant interpretation noted above, Cassirer maintained that the requirement of generally covariant laws was a vindication of the transcendental ideality of space and time, not, indeed, as “forms of intuition” but as “objectifying conditions” that further “de-anthropomorphized” the concept of object in physics, rendering it “purely symbolic”. The fundamental concept of object in physics no longer pertains to particular entities or processes propagating in space and time but rather to “the invariance of relations among (physical state) magnitudes” represented by generally covariant mathematical objects (tensors). For this reason, Cassirer concluded, the general theory of relativity exhibits “the most determinate application and carrying through within empirical science of the standpoint of critical idealism” (1921 [1957: 71; 1923: 412]).