 Those possibilities are mutually exclusive in the sense that no two of them can hold together; and they are jointly exhaustive in the sense that at least one of them must hold. A person can be more or less confident that a given possibility holds. Suppose that it makes sense to say of a person that she is, say, 80% confident that a certain possibility holds. In this case, say that this person’s degree of belief, or credence, in that possibility is equal to 0.8. A credence might be any other real number. (How to make sense of real-valued credences is a major topic for Bayesians, to be discussed in  §1.6  and  §1.7  below.)