 In 1979d Salmon discussed the propensity interpretation put forward by Popper to solve the single case problem arising within quantum mechanics. The idea is that probability should be taken as a dispositional property of the experimental set-up, or the generating conditions of experiments, liable to be reproduced over and over again to form a sequence (Popper 1959). Salmon argued that Popper’s proposal fares no better than the frequentist way of attaching probability values to single occurrences of events. In fact, the difficulties faced by frequentists in connection with identifying homogeneous reference classes are displaced rather than solved by the propensity account, which requires completeness of information in order to describe the chance set-up generating propensities.