 In the universities of Imperial and early Weimar Germany, the philosophy of Kant, particularly the various neo-Kantian schools, held pride of place. Of these, the Marburg School of Hermann Cohen and Paul Natorp, including Cohen’s student Ernst Cassirer, exhibited a special interest in the philosophy of the physical sciences and of mathematics. Yet prior to the general theory of relativity (1915–1916), Kantian philosophers accorded relativity theory only cursory attention. This may be seen in two leading Marburg works appearing in 1910, Cassirer’s Substanzbegriff und Funktionsbegriff and Natorp’s Die Logischen Grundlagen der Exakten Wissenschaften. Both books conform to the characteristic Marburg modification that greatly extended the scope of Kant’s Transcendental Logic, bringing under “pure thought” or “intellectual forms” what Kant had sharply separated in a distinction between the purely passive faculty of sensibility and the active faculties of understanding and reason. Of course, this revisionist tendency greatly transforms the meaning of Kant’s Transcendental Aesthetic and with it Kant’s conviction that space and time were forms of sensibility or pure intuitions a priori and so as well, his accounts of arithmetic and geometry. As will be seen, it enabled Cassirer, some ten years later, to view even the general theory of relativity as a striking confirmation of the fundamental tenets of transcendental idealism. In 1910, however, Cassirer’s brief but diffuse discussion of “the problem of relativity” mentions neither the principle of relativity nor the light postulate nor the names of Einstein, Lorentz or Minkowski. Rather it centers on the question of whether space and time are aggregates of sense impressions or “independent intellectual (gedankliche) forms”. Having decided in favor of the latter, Cassirer goes on to argue how and why these ideal mathematical presuppositions are necessarily related to measurable, empirical notions of space, time, and motion (1910: 228–9 [1923: 172–3]).