As late as in 1917 Reichenbach was still nearly an orthodox Kantian; ¹ three years later in his book Relativitätstheorie und Erkenntnis Apriori he resolutely rejected, or at least profoundly modified Kantian apriorism. What happened in the interval between 1917 and 1920 is not difficult to guess: the spectacular confirmation of the general theory of relativity in May 1919, when Einstein's prediction of the curvature of the light rays in the gravitational field of the sun was fully confirmed, attracted the attention not only of physicists and philosophers but of the general public as well. The theory of relativity was, using Gaston Bachelard's expression, "born of an epistemological shock" ² experienced when various experiments showed the impossibility of detecting any influence of the motion of the earth on the propagation of light. But "an epistemological shock" caused by experimental confirmations of the same theory was no less severe, especially among Kantian and neo-Kantian philosophers who were so profoundly imbued with the ideas of the Newtonian physics. It is then hardly surprising that Reichenbach's philosophical orientation was radically changed by the impact of the relativity theory.