a large literature has developed concerning variants of the problem. the standard assumption about the way the envelopes are set up is that a sum of money is in one envelope, and twice that sum is in another envelope. one of the two envelopes is randomly given to the player ("envelope a"). the originally proposed problem does not make clear exactly how the smaller of the two sums is determined, what values it could possibly take and, in particular, whether there is a minimum or a maximum sum it might contain. however, if we are using the bayesian interpretation of probability, then we start by expressing our prior beliefs as to the smaller amount in the two envelopes through a probability distribution. lack of knowledge can also be expressed in terms of probability.