Probability Assignments and the Principle of Indifference. An Examination of Two Eliminative Strategies

Abstract

The Principle of Indifference (PI) – roughly, the claim that equipossibility entails equiprobability – has long been regarded with suspicion by philosophers and scientists alike. This hostility has a twofold motivation. On one hand, there are the well-known inconsistencies (Bertrand paradoxes) to which the use of the principle leads. On the other, there are some general metaphysical and epistemological worries regarding the principle’s support for the idea that the occurrence of events in the physical world should follow ‘the directives of human ignorance’, as Reichenbach ([1949]/1971, 354) put it. These concerns naturally led philosophers to design strategies to eliminate PI from probabilistic reasoning. In this paper I examine two such strategies and argue that they are not successful. This conclusion, however, should not count so much as an endorsement of PI, but rather as an indication that another approach to explicating its role in probabilistic reasoning is needed.