 Reichenbach’s allegiance to Kantian formulations waxed and waned and transformed over time, but while the terminology changed, Reichenbach long retained the essential claim of his doctoral thesis that the establishment of any empirical law requires a superempirical presupposition about probability. In 1920, the argument and conclusions of the thesis were repeated with little qualification or variation in two essays, “The Physical Presuppositions of the Calculus of Probability”, and “A Philosophical Critique of the Probability Calculus”. Again in “Causality and Probability”, in 1930, Reichenbach reprises the claims of his doctoral thesis, but substitutes the “principle of induction”—that observed frequencies will continue to hold in new cases—for the “principle of probability.” The principle of induction is not given an explicitly a priori basis, the possibility of which Reichenbach had come to doubt when studying with Einstein (see Reichenbach, 1920f). Instead, it is justified by an uneasy mixture of loose convergence arguments (foreshadowing the straight rule, see below) and psychological habit. Without attribution, he dismisses a “conventionalist” account of the principle of induction, expressed in nearly Kantian terms, as “the principle of induction is not a statement about the physical world, but merely constitutes an ordering principle of science.” He argues that such an account does not “justify” scientific preferences for simpler hypotheses (1930g, see Reichenbach 1978, vol. II, p. 340–341). Arbitrary convention could choose any scientific hypothesis that captures the phenomena, but a descriptively accurate account of theory selection in science must explain why simpler theories should be preferred. The justification of the preference for simpler hypotheses remains unexplained in “Causality and Probability”, and Reichenbach’s own conclusion seems to be essentially the Kantian principle he rejects, supplemented with an attempt to prove that the principle of induction is unchallengeable because, in his view, probability claims are generalizations over as yet unobserved cases and therefore presuppose the principle of induction: 