One might ask Bob, what is the measure of Universes in which a Bob finds M but heeds 
M' without being suicidal or at least hazardous to his own health? At any rate, Bob 
could hardly have reached in sound mind & body the cognitive height of many-worlds 
ideas without heeding M instead of M'. "Why not stick with them's what brung you, 
Bob?" (Also, it seems to me that Bob might well have to be at least partially heeding 
familiar M in order to function well enough to try to heed M'.)

Of course, Bob could quip, "Denial is not just a river in Egypt." To some extent, we 
all stick to one or another interpretation in spite of the interpretation's apparent 
incoherence, & in spite of apparent contrary evidence. The fact that the incoherence 
or contrariness may be merely apparent is the temptation (we revise core ideas more 
reluctantly than others, & rightly so) -- the temptation to go wrong & persist even 
against a wind of disconfirming information. There is no surefire formula to avoid 
errors in any of these directions. There's open-mindedness (good) & there's flakiness 
& wishy-washyness (bad.) There's respect for what has stood the test of time so far 
(good) & there's dogmatism (bad). (As a practical matter, as regards knowingly to play 
by a different set of rules than that of the reality which one lives -- it seems to me 
that psychologically & biologically we are constituted so that we can't do that unless 
we are insane, & unlikely to be argued back to sanity.!

- Ben Udell

Hal Finney writes:

>Wei Dai writes:
>> Now suppose that two people, Alice and Bob, somehow agree that a measure M is the 
>> objectively correct measure, but Bob insists on using measure M' in making 
>> decisions. He says "So what if universe A has a bigger measure than universe B 
>> according to M? I just care more about what happens in universe B than universe A, 
>> so I'll use M' which assigns a bigger measure to universe B." What can Alice say to 
>> Bob to convince him that he is not being rational? I don't see what the answer 
>> could be.

>How about if she whacks him on the head?  Maybe that would knock some sense into him.

>Seriously, she could confront him with the reality that in the universe branch they 
>are in, measure M works, while M' does not.  Reality, whether in the form of a knock 
>on the head or more peaceful interactions, is not subjective.

>Now, true, there would be branches in the multiverse where M' worked while M did not. 
> Believers in objective measure would say that those branches are of low measure and 
>so "don't matter", but as you point out, Bob can argue symmetrically that this branch 
>where he is stuck with Alice and M has worked is also, to him, of low measure.

>But we can solve this conundrum while retaining symmetry.  Rationality should demand 
>allegience to the observed measure.   It is irrational to cling to a measure which 
>has been rejected repeatedly by observations. If classical definitions of rationality 
>don't have this property, we should fix them.  Bob is irrational to hold to M' in a 
>universe whose observations reveal M.

>Now, this will demand that in White Rabbit universes, ones where the quantum or 
>thermodynamic laws just happen to fail due to bad luck, a rational person would have 
>to abandon his (correct!) belief in a lawful universe and come to believe 
>(incorrectly!) in miracles.  However this is actually a reasonable requirement, since 
>we are stipulating that such
miracles have been observed.

>Hal Finney

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