denote again the amount of money in the first envelope by "a" and that in the second by "b". we think of these as random. let "x" be the smaller of the two amounts and "y=2x" be the larger. notice that once we have fixed a probability distribution for "x" then the joint probability distribution of "a,b" is fixed, since "a,b" = "x,y" or "y,x" each with probability 1/2, independently of "x,y".