against UD+ASSA, part 1

From: Wei Dai <>
Date: Wed, 26 Sep 2007 05:39:51 -0700

I promised to summarize why I moved away from the philosophical position
that Hal Finney calls UD+ASSA. Here's part 1, where I argue against ASSA.
Part 2 will cover UD.

Consider the following thought experiment. Suppose your brain has been
destructively scanned and uploaded into a computer by a mad scientist. Thus
you find yourself imprisoned in a computer simulation. The mad scientist
tells you that you have no hope of escaping, but he will financially support
your survivors (spouse and children) if you win a certain game, which works
as follows. He will throw a fair 10-sided die with sides labeled 0 to 9. You
are to guess whether the die landed with the 0 side up or not. But here's a
twist, if it does land with "0" up, he'll immediately make 90 duplicate
copies of you before you get a chance to answer, and the copies will all run
in parallel. All of the simulations are identical and deterministic, so all
91 copies (as well as the 9 copies in the other universes) must give the
same answer.

ASSA implies that just before you answer, you should think that you have
0.91 probability of being in the universe with "0" up. Does that mean you
should guess "yes"? Well, I wouldn't. If I was in that situation, I'd think
"If I answer 'no' my survivors are financially supported in 9 times as many
universes as if I answer 'yes', so I should answer 'no'." How many copies of
me exist in each universe doesn't matter, since it doesn't affect the
outcome that I'm interested in.

Notice that in this thought experiment my reasoning mentions nothing about
probabilities. I'm not interested in "my" measure, but in the measures
of the outcomes that I care about. I think ASSA holds intuitive appeal to
us, because historically, copying of minds isn't possible, so the measure of
one's observer-moment and the measures of the outcomes that are causally
related to one's decisions are strictly proportional. In that situation, it
makes sense to continue to think in terms of subjective probabilities
defined as ratios of measures of observer-moments. But in the more general
case, ASSA doesn't hold up.

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Received on Wed Sep 26 2007 - 08:41:17 PDT

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