 Bayes rule and the probabilities of theories posed another problem pressed on Reichenbach by Ernest Nagel in a review in Mind of the German edition of Reichenbach’s The Theory of Probability (Nagel, 1936; see also Nagel, 1938). Nagel’s courteous review found a series of difficulties with Reichenbach’s theory, beginning with the logical status of the “probability implication” which Nagel thought was not, contrary to Reichenbach, of the same kind as material implication, because Reichenbach’s relation is not “extensional.” Nagel does not explain what he means, and Reichenbach’s extended response in 1939 does not clarify the matter (1939b, 1978, vol. 2, p. 388). In his response to Popper (1935e), Reichenbach had proposed two ways of evaluating theories, one of which is to count the relative frequency of true statements among the consequences of a theory. Nagel points out that a theory 10% of whose tested predictions are false would scarcely have probability 0.9. Reichenbach does not attempt to defend this proposal in his reply to Nagel in 1938 (Reichenbach, 1938a). That leaves the assignment of probabilities to theories by Bayes Rule, which Reichenbach had also proposed, but Nagel observes that if theories are to be evaluated by Bayes rule they must have prior probabilities. How can there be a frequentist prior probability for a theory, since we do not know to what reference class to assign a particular theory and, whatever the reference class, according to Reichenbach we do not and cannot know which theories in that class are true? As in several of his other responses to criticism, Reichenbach’s 1938 reply is unfortunately more defensive than enlightening. About the reference class, Reichenbach says only “I do not think this is a serious difficulty as the same question occurs for the determination of the probability of single events” and recommends the choice of the narrowest reference class for which there are adequate statistics (though how the adequacy of a statistic for the truth or falsity of theories is assessed is never explained). Reichenbach gave a sketch of such a procedure in his 1935 reply to Popper, which Nagel pointedly does not think addresses the problems. About the problem of the frequency of true hypotheses in the reference class, Reichenbach says that we only need the probabilities of theories in the reference class, not their truth or falsity—a question-begging reply that Reichenbach must have sensed is unconvincing because he promises to address the issue further in Experience and Prediction (1938c). The discussion of the prior probabilities of theories in section 43 of Experience and Prediction is unsatisfactory for the same reasons. 