Mathematical Conundrum or Not? Number Six What do you mean by 'exhaustive disjunctive possibilities'? — andrewk
Exhaustive disjunctive possibilities are those that are mutually exclusive and jointly exhaustive, viz. those that form a partition over the space of possible outcomes.
That's all correct, but is not inconsistent with my note. Nowhere does it say that 5 and 20 are X and 2X, given we have seen that Y=10. Rather, we know that X is either 5 or 10, so either
we have X in the envelope we opened, so that X = Y = 10; OR
X is in the other envelope, so we have Y = 2X = 10 and X=5 is in the other envelope. — andrewk
No, this doesn't work. If you
define Y in terms of X, then everything you write with Y can (must) be rewritten in terms of X. Do this and the illusion that Y is a value you can average over multiple situations disappears. You are acting like Y is some other value that you can now double or half in terms of what you draw. But it is
not: it is, for some
fixed X, either X or 2X. It follows that whatever the hidden amount is, the two possibilities over which we average must be such that one is double the other. 5, 20 violates this and so cannot be what we average over – it is
not consistent with the way the problem is set up, because we know there is some value of X fixed at the time of the choosing of the two envelopes, and the only possibilities are that the hidden amount is either X or 2X,
regardless of what amount you look at in one envelope. The fact that you even see one envelope at all is a complete red herring, only thrown into the problem to seduce you into making this very fallacy.