 Salmon followed Reichenbach in embracing an objective version of Bayesianism according to which prior probabilities should be determined on the basis of objective and empirical criteria. Prior probabilities represent the “best estimates of the frequencies with which certain kinds of hypotheses succeed” (Salmon 1990d [2005: 102]), and must be fixed by means of (objective) criteria such as simplicity, symmetry and analogy. Analogy, in particular, is apt to suggest comparisons with other similar theories, whose rate of success in the past can be assessed. Salmon conceded that priors are sometimes the expression of rough estimates, and can be stated in terms of interval probabilities. In any case, he regarded prior probabilities as an essential component of hypothesis confirmation, as testified by many cases offered by the history of science. The fruitfulness of assuming a Bayesian perspective in studying the history of science is strongly emphasized: