 Both of these papers rely on the idea that conditionalisation is a good way to update beliefs. Neither, by the way, rely on the idea that conditionalisation is the only rational way to update beliefs; the arguments go through given merely the permissibility of conditionalising. Many Bayesians hold something stronger, namely that conditionalisation is the way to update beliefs. One widely used argument in favour of this position is a so-called ‘Dutch Book’ argument. This argument shows that if you plan to follow any strategy for revising beliefs other than conditionalisation, and you do follow that strategy, then someone who knows the strategy that you’re going to follow can produce a series of bets that will seem favourable to you when each is offered, but which will collectively lead to a sure loss. It is also true that if you do update by conditionalisation, there is no similar Dutch Book that can be made, but Lewis did not prove this. (It is Theorem V in Skyrms (1987).) The Dutch Book argument for conditionalisation was introduced to the literature by Paul Teller (1976), who credited it to Lewis. Lewis’s own version of the argument did not appear until 1999, in Papers in Metaphysics and Epistemology, under the title “Why Conditionalize?” (1999b). This was something he had written as a course handout in 1972, and which had been very widely circulated, and, via Teller’s paper, very influential on the development of Bayesian epistemology.