 for \(i \ne j\). Such a procedure is the content of a rule of multiple homogeneity which “expresses the fundamental condition for adequate explanation of particular events” (Salmon 1970b [1971: 59]). Fulfillment of this rule guarantees against the inclusion of irrelevant information in the explanatory account. Once the process leading to the specification of a homogeneous reference class is completed, the event to be explained is associated with a probability distribution. The shift from a non-homogeneous to a homogeneous reference class embodies the explanatory power of the S-R model, according to which