But I guess the problems in this discussion is the lack of precise
definition of the terms and of the philosophical framework.

This is where I most often feel like speaking up on this amazing list.
I don't have enough math to really understand things like the Speed Prior, etc., but I do think
that there are hidden philosophical assumptions (about consciousness, especially) behind many of the approaches here.

First, in this discussion I am always assuming MWI.

In a materialistic framework - with nothing external to the
physical world - it is hard to define personal identity if we
take the MWI in account.

I agree; and in fact it is hard no matter what our favorite metaphysical theory says, MWI or otherwise. "I" is a very tough philosophical problem.

But in this case there is clearly
no 'soul' or anything other than the configuration of atoms
to describe what we call 'ourselves'. In any branching of
the multiverse there are multiple copies of my body being
produced. Nevertheless, I only experience one of those
states. Therefore, I guess the best I could say is that ' I '
is one of the instances of this configuration.

When it becomes difficult to say things about "I", I find it helps to attempt to cast the entire situation into 3rd-person terms. I would say here that under MWI (and UD, and the rest..) there is a ensemble of entities more or less similar to Eric. Some of these are different enough that they could be considered different people (such as the Eric who inherited a billion dollars when he was 12 years old), some of them quite similar (such as the Eric who lives where the ambient room temperature is 2 degrees colder) and some of them are so similar (different only temporarily or unnoticeably, or at the quantum level) that they can't be told apart. "Individual Eric"s within the ensemble can be seen to actually each consist in a fuzzy set of indistinguishable Erics. The "boundaries" between sets demarcate where differences at the finest-grained levels start to make a difference at levels that an Eric can notice, given the kinds of observations he's making. The boundaries being "fuzzy" means just that there is no fact of the matter which other copies are really "you" and which are not. This is not a problem! Well, it is a problem if you insist on some definition of "I", like that of Descartes, that rules this out; then you must argue for that definition.

Let me stress this point: *I am, for all practical purposes,
one and only one specific configuration of atoms in a
specific universe. I could never say that ' I ' is ALL the
copies, since I NEVER experience what the other copies
Here I think you're making an assumption. You are certainly not ALL the copies, but then it doesn't follow that you are only 1. "You" could be a fuzzy set of copies that have experiences so similar that they cannot be told apart. That is, they cannot be told apart >yet<. Unnoticeable differences eventually can percolate up and make a noticeable difference, or they can be made noticeable by making more sensitive observations.
In some of these branching universes, this configuration
of atoms that I call 'me' will not show signs of what we
call life anymore. Notice that death is no different from
any other branching in the multiverse in a materialistic
point of view. There is no 'soul' being detached from the
body or anything else. So there is no reason to suppose
that my personal experiences will not be, as before, one
of any of the future configurations of these atoms that I
call 'me', including those where this configuration is a
'dead' state.
In particular, after a severe car crash, most of these will
be dead. Notice again that 'dead' has, in this paradigm,
no supernatural meaning, it means nothing more than 'that
body does not show vital functions anymore'. In particular,
that body has no sensorial experiences anymore. But there
is yet no reason to suppose that I cannot be one of those
bodies. Therefore, in this framework, in the case of a severe
car crash, the probability that I have no more future sensorial
experiences - i.e., that I am dead for good (or bad?) - is
simply the measure of universes where my body is dead.

When some people suppose that our next experience is
necessarily one of the alive ones, they are tacitly assuming
a dualistic position.
I think they are indeed making an assumption, but not necessarily a dualistic one. They are making the assumption that there is always a "way out" - that in every set of "I"'s who encounter a dangerous situation, there is always at least one that has a continuation. There is no need to postulate some "soul" leaping from one body-copy to another.

If a large set of Eric-equivalents encounter a really dangerous situation, most will not continue, but as long as this assumption holds:

"(The set of reasonably similar Eric-equivalents) contains
(The set of Erics who are unnoticeably different from you) which contains
(The set of Erics who have a living continuation after event X)
which has at least one member."

then you will not experience yourself dying. I think this is how materialism can accomodate QTI. I do think a better attack on QTI is that the final part of the above assumption (the last set has at least one member) isn't well-argued for. Even if these Eric-sets are infinite there may not be an Eric who survives, say, the sun exploding; just as the infinite set of composite numbers doesn't contain any primes.

But if we decide to accept a dualistic framework QTI would
probably be the least probable scenario. We could as well say
that the next experience would be of many other kinds: in other
bodies, reincarnation, or any transcedental experience like
going to heaven - there is no reason to decide between these.
I think this reduces to the white rabbit problem again. The next experience could *always* be of many other kinds, whether we've just had a dangerous event or not. I think it is an interesting question though, for surviving a fall from a cliff, say, certainly seems like a "white rabbit" event. Perhaps one would conjecture that under QTI, "white rabbit" events that allow incredible survival must still be as mediocre as possible.

For instance, QTI poses a difficulty for the dualist: at each
moment, if QTI and is true, an infinity of 'souls' is merging into
one single body, since this body is dying at an infinity of other
universes. How does this square with the common definition
of a 'soul' as an immaterial *individuality*?

I think this is the least of dualism's problems..=)

Best regards,
Pete Carlton, Ph. D.
Life Sciences Division
Lawrence Berkeley National Lab
Berkeley, CA USA

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