  Philosophers concerned with the interpretation of probability are usually dealing with the following problem: Probability is characterized by a number of formal rules, the additivity of probabilities for disjoint sets of possibilities being the most central of these. But what ought we to take the formal theory to be a theory of? Some interpretations are “objectivist,” taking probabilities to be, possibly, frequencies of outcomes, or idealized limits of such frequencies or perhaps measures of “dispositions” or “propensities” of outcomes in specified test situations.