Explanatoriness and Evidence: A Reply to McCain and Poston

We argue elsewhere that explanatoriness is evidentially irrelevant (Roche and Sober 2013). Let H be some hypothesis, O some observation, and E the proposition that H would explain O if H and O were true. Then O screens‐off E from H: Pr(H | O & E) = Pr(H | O). This thesis, hereafter “SOT” (short for “Screening‐Off Thesis”), is defended by appeal to a representative case. The case concerns smoking and lung cancer. McCain and Poston grant that SOT holds in cases, like our case concerning smoking and lung cancer, that involve frequency data. However, McCain and Poston contend that there is a wider sense of evidential relevance—wider than the sense at play in SOT—on which explanatoriness is evidentially relevant even in cases involving frequency data. This is their main point, but they also contend that SOT does not hold in certain cases not involving frequency data. We reply to each of these points and conclude with some general remarks on screening‐off as a test of evidential relevance.