 Popper (1957) is motivated by the desire to make sense of single-case probability attributions that one finds in quantum mechanics—for example ‘the probability that this radium atom decays in 1600 years is 1/2’. He develops the theory further in (1959a). For him, a probability \(p\) of an outcome of a certain type is a propensity of a repeatable experiment to produce outcomes of that type with limiting relative frequency \(p\). For instance, when we say that a coin has probability 1/2 of landing heads when tossed, we mean that we have a repeatable experimental set-up — the tossing set-up — that has a propensity to produce a sequence of outcomes in which the limiting relative frequency of heads is 1/2. With its heavy reliance on limiting relative frequency, this position risks collapsing into von Mises-style frequentism according to some critics. Giere (1973), on the other hand, explicitly allows single-case propensities, with no mention of frequencies: probability is just a propensity of a repeatable experimental set-up to produce sequences of outcomes. This, however, creates the opposite problem to Popper’s: how, then, do we get the desired connection between probabilities and frequencies?