This puzzle has stayed with me for a decades for two reasons. #1 is it was the first time I was embarrassed in public discourse on Usenet. Guy shot me down pr

This puzzle has stayed with me for a decades for two reasons. #1 is it was the first time I was embarrassed in public discourse on Usenet. Guy shot me down pretty hard, but when he was done there was no uncertainty- he was right and I was wrong. #2 is I finally realized the importance of that postulate of Quantum Mechanics: The wavefunction contains everything you can know about the system. This problem pointed out for me that probability is not an unvariable thing. Different people will calculate different probabilities for different events (think if we're playing cards and I have three aces and you have none- you will calculate a different probability of drawing a ace that I will) and we will both be correct. This without this postulate the probabilities of QM would be meaningless.