> 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.