In the article above there is a serious error I think. Suppose we open an envelope and find $10 inside. According to the section "What if you open envelope A?" once you've opened an envelope, you should always switch. Furthermore it is argued that this decision would be empirically justified by the results we would find when carrying out the experiment repeatedly. This defies common sense. All we know is how much is in one envelope and that one envelope contains twice as much as the other. The probability of getting the larger amount by switching is 50% regardless of whether we have opened the envelope or not. I can clarify what I mean in the following way. Suppose we carry out the experiment a hundred times each with two envelopes. Supposed each experiment is identical. We open one envelope in one experiment and find it has$10. There are two possible scenarios. Scenario A is that in every experiment one envelope contains $10 and the other contains$20. Scenario B is that in every experiment one envelope contains $10 and the other contains$5. If Scenario A is the actual experimental setup, we should always switch. If Scenario B is the actual setup, we should always stay. But we don't know if we're in Scenario A or Scenario B.