Why explanatoriness is evidentially relevant

William Roche and Elliott Sober argue that explanatoriness is evidentially irrelevant. This conclusion is surprising since it conflicts with a plausible assumption—the fact that a hypothesis best explains a given set of data is evidence that the hypothesis is true. We argue that Roche and Sober?s screening-off argument fails to account for a key aspect of evidential strength: the weight of a body of evidence. The weight of a body of evidence affects the resiliency of probabilities in the light of new evidence. Thus, Roche and Sober are mistaken. Explanatoriness is evidentially relevant. We are explanationists. That is, we think that the fact that a hypothesis best explains some data is evidence that the hypothesis is true. This is a basic commitment of anyone who makes use of inference to the best explanation (IBE). Given the ubiquity of IBE in everyday life and the sciences, we find it surprising that William Roche and Elliott Sober (Roche and Sober 2013) have recently attempted to show that explanatoriness is evidentially irrelevant. Roche and Sober (Roche and Sober 2013, 659) claim that if explanatory considerations are evidentially relevant, then when considering some hypothesis H and some observation O, the proposition E, 〈If H and O were true, H would explain O〉 should raise the probability of H. The notion of ‘probability’ here is epistemic probability, also referred to as ‘rational credence’. In other words, they claim that explanatoriness is positively evidentially relevant if and only if Pr (H|O&E) > Pr ∗Thanks to Kenny Boyce, Branden Fitelson, Jon Matheson, William Roche, and Elliot Sober for helpful comments on an earlier draft. We are also grateful for comments from two anonymous reviewers which led to significant improvements in our arguments. See (Glymour 1984; Thagard 1978) for discussion of the prevalence of IBE in our everyday reasoning and examples of it being employed in the sciences.