 For Salmon, the fundamental role played by the principle of the common cause in connection with realism was testified by a great many examples in the history of science, his favorite being Perrin’s ascertainment of Avogadro’s number around 1912. As recounted by Mary Jo Nye in Molecular Reality (1972), after Perrin showed that Avogadro’s number could be determined by means of thirteen different methods, all producing results in close agreement, the community of physicists was convinced of the reality of atoms and molecules. According to Salmon, Perrin’s conclusion was reached by means of an inductive reasoning which “can appropriately be schematized as a type of common cause argument” (Salmon 1985b [2005: 17]). From this and other examples Salmon concluded that a powerful method for inferring hypotheses from empirical evidence of regularities, the principle of the common cause provides a tool for inferring unobservables from observables.