The mathematics of probability enable us to derive some probability judgements from others. For example, if we are given the probability of an event of some kind occurring on some occasion, we can derive the probability that the frequency of the event on a succession of occasions falls within some interval. An inverse method is an attempt to derive probabilities themselves purely from observed frequencies of events. The possibility of inverse methods is the subject of correspondence between Jakob Bernoulli (1654–1705) who was optimistic about such methods, although not in the Ars Conjectandi, and Leibniz (sceptical). The most notorious example of an inverse argument is Laplace’s rule of succession.