INTRODUCTION

John is a bookie who posts odds on sporting events. In determining the probability that the Dallas Cowboys will defeat their next opponents, John takes into account the various strengths and weaknesses of both teams. John then declares that this probability is 0.5. But at this point John learns that another bookie, Trish, has declared that the probability of a Cowboy victory is 0.01 – John is surprised, for he holds Trish's opinions in such matters in high regard. John may feel that he should modify his own posted probability on the basis of knowing the probability that Trish has posted. But how, exactly, should he go about assimilating this information? John's predicament raises the general problem of how rational agents do or should make use of other persons' posted probabilities as evidence. How, if at all, is John to combine the content of his own deliberations about the result of the Cowboys' game with the information about Trish's probability to compute an updated probability?

Philosophers have differed over whether scientists (and rational agents generally) do or ought to consider other persons' probabilities as epistemically significant. Earman argues (1993: 30) that “It is fundamental to science that opinions be evidence-driven” rather than driven by reference to other persons' opinions.