What to do (2)

This is a continuation of last Friday's post.  There is a logical technique for testing an argument known as reductio ad absurdum whereby if an argument leads to an impossible or absurd result then that is proof that the argument is false.  You have probably realised that our reasoning in the "two envelopes" problem leads to the absurd result that, whichever envelope we choose, we can use that reasoning to show that the other envelope must contain 25% more money - so we would swap from envelope 1 to envelope 2, and then swap back to envelope 1 then back to envelope 2 and so on ad infinitum.  So our reasoning is clearly false, but why? That's not so easy to get your head round - see if you can get it.  All will be revealed next time.