[I]t is clear that some things are almost certain, while others are matters of hazardous conjecture. For a rational man, there is a scale of doubtfulness, from simple logical and arithmetical propositions and perceptive judgments, at one end, to such questions as what language the Myceneans spoke or “what song the Sirens sang” at the other…. [T]he rational man, who attaches to each proposition the right degree of credibility, will be guided by the mathematical theory of probability when it is applicable…. The concept “degree of credibility,” however, is applicable much more widely than that of mathematical probability.