If you have a mutually exclusive and exhaustive set of propositions , each of which specifies a plausibility for the one proposition B you're interested in, then your total plausilibity is . (Actually this is true whether or not the A's say anything about B. But if they do, then this can be useful way to think about P(B).)

I haven't said how to assign plausibilities to the A's (quick, what's the plausibility that an unspecified urn contains one white and three cyan balls?), but this at least should describe how it fits together once you've answered those subproblems.