 Probability is a function, P, that assigns values between zero and one, inclusive. Usually the arguments of the function are taken to be sets, or propositions in a formal language. The formal term for these arguments is ‘events’. We will here use the notation that is appropriate for propositions, with ‘\(\nsim\)’ representing negation, ‘&amp;’ representing conjunction, and ‘\(\vee\)’ representing disjunction. Sometimes when there is a long conjunction, this is abbreviated by using commas instead of ampersands. The domain of a probability function has the structure of a field or a Boolean algebra. This means that the domain is closed under complementation and the taking of finite unions or intersections (for sets), or under negation, conjunction, and disjunction (for propositions). Thus if A and B are events in the domain of P, so are \({\nsim}A\), \(A \amp B\), and \(A \vee B\).