the simple resolution depended on a particular interpretation of what the writer of the argument is trying to calculate: namely, it assumed he was after the (unconditional) expectation value of what's in envelope b. in the mathematical literature on two envelopes problem a different interpretation is more common, involving the conditional expectation value (conditional on what might be in envelope a). to solve this and related interpretations or versions of the problem, most authors use the bayesian interpretation of probability, which means that probability reasoning is not only applied to truly random events like the random pick of an envelope, but also to our knowledge (or lack of knowledge) about things which are fixed but unknown, like the two amounts originally placed in the two envelopes, before one is picked at random and called "envelope a". moreover, according to a long tradition going back at least to laplace and his principle of insufficient reason one is supposed to assign equal probabilities when one has no knowledge at all concerning the possible values of some quantity. thus the fact that we are not told anything about how the envelopes are filled can already be converted into probability statements about these amounts. no information means that probabilities are equal.