 If E is an old piece of evidence that a person had received before, this person’s credence in E is currently 1. So, conditionalization on E at the present time would involve dropping no credence, followed by rescaling credences with a factor of 1—so there is no credence change at all. Then, by the Bayesian account of confirmation stated above, old evidence E must fail to confirm new theory H. But that result seems to be wrong because the old data about the advance of Mercury’s perihelion confirmed Einstein’s new theory; this is Glymour’s problem of old evidence, construed as a challenge to a Bayesian account of confirmation. But, if Earman (1992) is right, the Mercury Case challenges not just Bayesian confirmation theory, but actually cuts deeper, all the way to one of the two core Bayesian norms—namely, the Principle of Conditionalization—as suggested by Earman’s problem of old evidence. For attempted solutions to Earman’s old evidence problem (about conditionalization), see  section 5.1  below. For more on Glymour’s old evidence problem (about confirmation), see  section 3.5 of the entry on confirmation.