 By the Principle of Indifference, the answers seems to be 0.5 and 0.5, respectively. If so, there should be equal credences (namely 0.5) in A and in B. But this result sounds wrong to Keynes. He thinks that, compared with urn a, we have much less background information about urn b, and that this severe lack of background information should be reflected in the difference between the doxastic attitudes toward propositions A and B—a difference that the Principle of Indifference fails to make. If so, what is the difference? It is relatively uncontroversial that the credence in A should be 0.5, being the ratio of the white balls in urn a (perhaps thanks to the Principal Principle). On the other hand, some Bayesians (Keynes 1921; Joyce 2005) argue that the credence in B does not have to be an individual real number but, instead, is at least permitted to be unsharp, being the interval \([0, 1]\), which covers all the possible white-to-black ratios under consideration. This is only one motivation for an interval account of unsharp credences; for another motivation, see  supplement A.