 Example (Two Urns). Suppose that there are two urns, a and b. Urn a contains 10 balls. Exactly half of those are white; the other half, black. Urn b contains 10 balls, each of which is either black or white, but we have no idea about the white-to-black ratio. Those two urns are each shaken well. A ball is to be drawn from each. What should our credences be in the following propositions?