Jim Humphries Lewis's counterpart theory deals with the difficulty of transworld identity (one argument against possible worlds discourse is that the idea of a transworld individual is absurd, since it violates the principle of indiscernability of identicals). So far as I am aware the proponents of MWI have not considered this issue: they claim neither that inhabitants of other worlds are counterparts in Lewis' sense, nor that they are identicals ( nor would this seem to be a scientific question) - to that extent it is perhaps premature to say that counterfactuality is rendered untenable by MWI.

Ed Weinmann What varies, across worlds in the MWI, is the relative state of the observer. Any two worlds, in which the same observer is in the exact same relative state, are not two worlds, but the same world. Hence no two worlds, in MWI, can be exactly the same... the relative state of at least one observer, in each world, must differ from that in the other, be it by ever so little.

Vic Stenger I was meaning to use this to rebut your earlier message that talked about a "you" being in different worlds. Since no two worlds are like, they are different yous.

Ed Weinmann Owing to quantum entanglement, the physical system constituting the one world will differ from the physical system constituting the other. When the relative state of an observer splits- one seeing the cat alive, in one world, the other seeing the cat dead, in the other world- those two worlds will differ for ever more. Now notice this reflects the view that the real = the physical... not surprising, since Everett was a physicist... and this has consequences, in the area of what philosophical issues are going to arise... they will not be the ones of the traditional Platonic agenda!

Vic Stenger Still sounds Platonic to me, what "Tegmark called the view from outside" and admits is platonic. Most physicists are platonic on same matters, like the

reality of electromagnetic fields.

Ed Weinmann Given any physical difference whatever- say, in the quantum state of a single neutrino- you have another world. All the *rest* might be quite identical. So yes, there might exist an identical Ed Weinmann, in another world exactly identical to this one, but for a different series of oscillations in the quantum state of some neutrino, somewhere. The quantum state of the neutrino need not actually be observed by what we would consider a conscious observer, either, to make the difference, just as the outcome of the experiment with Schrodinger's Cat need not be observed, to make the difference and split off a "new" world.

Vic Stenger I will be saying more about this, but I like Tegmark's term "No Collapse Interpretation" for MWI, and his point that these "many worlds" (1) have the same wave function and (2) Can interfere with one another. They all constitute "the universe."

Ed Weinmann Hence, while the ontological claim of Everett's original MWI extended no further than the proposal as to relative states of the observer, it is clear that the only metaphysical distinctions it makes would be between worlds, as defined by the variance in those relative states, and not between identical individuals (physical subsystems of those worlds), which could be called identical, because indiscernible. Everett's MWI *does* involve metaphysics, but not the kind that would be involved in asserting the identity of indiscernibles... Everett's metaphysics recognized only worlds, and not individuals therein... which is why, as you mentioned above, the issue of the identity of indiscernibles does not arise in MWI discussions. I hope this shows how contemporary physical theory, with its new departure in quantum mechanics, has also brought about a new departure in philosophy. The old baggage train, which came along with the Platonic agenda, has been abandoned, and I consider this to be real progress.

James Higgo Deutsch has the opposing view: that there are very many identical people in different worlds, and that it does not make sense to say that they are other than identical, i.e. the same person. I agree with this. Any problem is with our workaday concepts of identity; I see no reason why it affects MWI.

Jim Humphries See my reply to Ed where I argue that in fact these individuals

have at least one attribute ( that of interacting with a unique universe which the others cannot interact with) which is distinctive , thus violating the principle of the indiscernibility of identicals.

Scott Dalton At this point, am I supposed to accept the other Scotts as part of my identity--the Nobel-prize winning physicist, the one behind bars, the Lotto winner and many others? This would make no sense--no one has paid or recognized me and no one has locked me up (yet). Then it must make better sense not to regard our counterparts in other universes as part of our identity. But are they real? Lloyd argues, roughly, since they have such low amplitudes (virtually orthogonal in "possibility space") relative to our branch, they are virtually non-existent and therefore not real, and in any case, not important. Deutsch argues that virtually non-existent is not non-existent, so they are real. It's as unproblematic as our classical view of identity--don't belabor it, accept it.

... I would be satisfied with a good explanation--including interpretation--of quantum mechanics. A big step would be determining whether other decohered branches have exactly zero amplitude (true wavefunction collapse) or small but measureable amplitudes (with respect to our "branch"). For now, I can accept that there are good theoretical reasons for assuming non-collapse and making sense of it is an interesting problem and crucial for a deep understanding of quantum theory and what it says about fundamental reality.

James Higgo MWI exposes problems with the concept of identity. Are you a 2-year-old? is a 2-years old part of you? We've always had these problems; in fact whole religions of billions have the denial of this concept at their core. There is a standard buddhist argument which shows that nothing permanent connects you and your past 'you'; the aim is to weaken the false concept of self. It is unsurprising, but disappointing, that so many people jump to blame MWI for their own difficulties with this difficult concept.

Jacques Mallah Nature must have a mathematical criterion for [human identity], if it is going to figure in a theory of physics.

James Higgo Which implies he has a divine right to figure in a theory for physics. No, you don't, Jacques. Human identity does not figure in any physical theory; it is merely a useful social construct.

Jacques Mallah I hope even you have realized by now that an argument against immortality quoted above is that if immortality were true, identity would

have to figure in physics. So you are supporting my point above, not

opposing it.

James Higgo Nonsense. What I am arguing is that we can expect to experience a continuous flow of consciousness forever. That does not mean we have an objective identity. I am arguing about our experience, which is 100% subjective. You seem unable to separate the issue of objective from subjective. Objectively, there is no structure to the universe; structure is only seen when you occlude your viewpoint by being subjective. This is Tegmark's major insight in 'Does the universe contain almost no information?' (BTW do you think that he and Schmidhuber make sense on this?)

James Higgo Jacques, nobody denies that our measure decreases with time. Do you deny that there is a billion-year Jacques somewhere out there in the infinite universe?

Jacques Mallah Are you trying to pass the disagreement off as some kind of semantics? It's not. Of course you know that I do believe there is a billion year old Jacques, so why are you asking that?

James Higgo (I assume you are also OK with the notion that there is an infinite number of billion-year-old-Jacqueses). Well, then our only disagreement is whether you will become an old Jacques. I believe that you only experience those universes in which you exist, so you will eventually experience being one of those old Jacqueses. Would you care to offer a non-SSA argument against this (SSA is inapplicable to this situation, as I have pointed out, as you are the only immortal and therefore not a representative sample).