Bruno wrote:

> I don't take the notion of observer for granted.

Neither do I, of course. The observer O is something computable that
evolves in some universe U.

> The problem is that "to be in a universe" has no clear meaning

But it does.  There is a computable predicate S such that S(U)=TRUE  if
"O inhabits U."  Fortunately, there is no need for us to further specify
what it means to ``inhabit,'' to be ``conscious'' or ``self-aware,'' or 
whether there is some other observer who applies S to U and uses S(U)
to identify O, etc.

Now we have a formally defined, conditional probability distribution
on all universes satisfying S.  I thought this to be clear, but maybe
I should have written it down explicitly.


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