 Reichenbach’s official definition of probability for infinite sequence pairs ⟨xi, yi⟩ with xi in A, and yi in B, for which the limit p of the relative frequency of B in A exists, is as follows: “the limit p is called the probability from A to B within the sequence pair.” (1949f, p. 69). Constraints on the nature of the sequences are added later (1949f, section 30) that are aimed to formalize aspects of randomness. Probabilities of single cases are “fictive” or elliptical, to be understood as claims about the frequency of a kind of case in an implicit reference class. Elsewhere, Reichenbach puts more emphasis on finite frequencies and even suggests that limiting frequencies are simply a mathematical device for justifying inductive procedures (‘A letter to B. Russell’, 1978, vol. II, p. 405–406). Once introduced, the logical framework is dropped in the mathematical development of probability theory in the book. 