Let us start by considering two formulations of the Monty Hall problem (Grinstead and Snell, Introduction to Probabilities). The rst formulation is: Suppose you are on a Monty Hall's Let's Make a Deal!You are given the choice of three doors, behind one door is a car, the others goats. You pick up a door, say 1, Monty Hall opens another door, say 3, which has a goat. Monty says to you Do you want to pick door 2? Is it to your advantage to switch your choice of doors? (Grinstead and Snell, Introduction to Probabilities, Example 4.6, p. 136) The second formulation is more general: We say that C is using the stay strategy if she picks a door, and, if oered a chance to switch to another door, declines to do so (i.e., he stays with his original choice). Similarly, we say that C is using the switch strategy if he picks a door, and, if oered a chance to switch to another door, takes the oer. Now suppose that C decides in advance to play the stay strategy. Her only action in this case is to pick a door (and decline an invitation to switch, if one is oered). What is the probability that she wins a car? The same question can be asked about the switch strategy. (Idem, p. 137) Grinstead and Snell remark that the rst formulation of the problem asks for the conditional probability that C wins if she switches doors, given that she has chosen door 1 and that Monty Hall has chosen door 3 whereas the second formulation is about the comparative probabilities of two kinds of strategies for C, the switch strategy and the stay strategy. They point out that using the stay strategy, the contestant C will win the car with probability 1/3, since 1/3 of the time the door he picks will have the car behind it. But if C plays the switch strategy, then he will win whenever the door he originally picked does not have the car behind it, which happens 2/3 of the time. (Idem, p. 13). Grinstead and Snell give a solution to the rst problem using conditional probabilities. van Benthem (2003) gives a solution to the same problem in terms of product updates and probabilites.