 A Dutch book is a series of bets bought and sold at prices that collectively guarantee loss, however the world turns out. Suppose we identify your credences with your betting prices. Ramsey notes, and it can be easily proven (e.g., Skyrms 1984), that if your credences violate the probability calculus, then you are susceptible to a Dutch book. For example, suppose that you violate the additivity axiom by assigning \(P(A \cup B) \lt P(A) + P(B)\), where \(A\) and \(B\) are mutually exclusive. Then a cunning bettor could buy from you a bet on \(A \cup B\) for \(P(A \cup B)\) units, and sell you bets on \(A\) and \(B\) individually for \(P(A)\) and \(P(B)\) units respectively. He pockets an initial profit of \(P(A) + P(B) - P(A \cup B)\), and retains it whatever happens. Ramsey offers the following influential gloss: “If anyone’s mental condition violated these laws [of the probability calculus], his choice would depend on the precise form in which the options were offered him, which would be absurd.” (1990, 78) The Dutch Book argument concludes: rationality requires your credences to obey the probability calculus.