Smith on Justification and Probability

Call Justificatory Probabilism (hereafter, JP) the thesis that there is some (classical) probability function Pr such that for an agent S with evidence E , the degree to which they are justified in believing a hypothesis H is given by Pr(H |E). As stated, the thesis is fairly ambiguous, though none of the disambiguations are obviously true. Indeed, several of them are obviously false. If JP is a thesis about how justified agents are in fully believing propositions, it is trivially false. I’m about to flip a penny. Call H the proposition that it will land heads. Right now I’m completely unjustified in believing either H or ¬H . Yet according to JP, at least one of them must be half-justified. Richard Holton (2008) has argued that full belief comes in degrees. This is an attractive thesis, especially on a pragmatic view of belief. Start with the picture that beliefs are, as Ross and Schroeder (fort) say, something like those propositions we are disposed to take as given in inquiry. Now in different inquiries we will take different things as simply given (Hawthorne, 2004). So we might measure the strength of a full belief as something like the range of inquiries in which we’ll take it as given. Then we could interpret JP as a thesis about how strongly S is justified in believing H . But again, it would be false, for the same reason as given in the previous paragraph. We are completely unjustified in taking as given either that a coin will land heads or that it will land tails, contra JP. It is more plausible to take JP as a thesis about credences, one that has consequences for the theory of belief given some connection between beliefs and credences. What thesis could it be? The following three candidates spring to mind.