if we don't look into the first envelope, then clearly there is no reason to switch, since we would be exchanging one unknown amount of money ("a"), whose expected value is infinite, for another unknown amount of money ("b"), with the same probability distribution and infinite expected value. however, if we do look into the first envelope, then for all values observed (formula_24) we would want to switch because formula_20 for all "a". as noted by david chalmers (2002), this problem can be described as a failure of dominance reasoning.