Epistemic probabilities are degrees of support, not degrees of (rational) belief

Abstract

AbstractI argue that when we use ‘probability’ language in epistemic contexts—e.g., when we ask how probable some hypothesis is, given the evidence available to us—we are talking about degrees of support, rather than degrees of belief. The epistemic probability of A given B is the mind‐independent degree to which B supports A, not the degree to which someone with B as their evidence believes A, or the degree to which someone would or should believe A if they had B as their evidence. My central argument is that the degree‐of‐support interpretation lets us better model good reasoning in certain cases involving old evidence. Degree‐of‐belief interpretations make the wrong predictions not only about whether old evidence confirms new hypotheses, but about the values of the probabilities that enter into Bayes' Theorem when we calculate the probability of hypotheses conditional on old evidence and new background information.