   Subjectivists invoke (2.1d) to explain why scientists so often regard improbable or surprising evidence as having more confirmatory potential than evidence that is antecedently known. While it is not true in general that improbable evidence has more confirming potential, it is true that E's incremental confirming power relative to H varies inversely with E's unconditional probability when the value of the inverse probability PH(E) is held fixed.  If H entails both E and E*, say, then Bayes' Theorem entails that the least probable of the two supports H more strongly.  For example, even if heart attacks are invariably accompanied by severe chest pain and shortness of breath, the former symptom is far better evidence for a heart attack than the latter simply because severe chest pain is so much less common than shortness of breath.