What exactly is the probability of an event? There are several ways to answer: the frequentist approach ties it to the frequency with which the event recurs over many trials, whereas the subjectivist approach (due to Bruno de Finetti) interprets it as the fair price of a bet. Both lead to the same formal rules, summarised in Kolmogorov’s axioms, from which the practical properties of the calculus follow: the probability of the complement and the probability of the union of two events.