I think if you fully taboo the concepts of “personal identity” and “existence”, the argument evaporates. Before tabooing, your argument looks like this:
Alternate universes, containing persons psychologically continuous with me, exist.
Persons psychologically continuous with me are me.
Therefore I am immortal.
Therefore I should anticipate never dying.
Therefore I should make plans that rely on never dying.
On the face of it, it seems sound. But after tabooing, we’re left with something like:
Our best, most parsimonious models of reality refer to unseen alternate universes and unseen persons psychologically continuous with each other.
In such models, alternate futures of a person-moment have no principled way of distinguishing one of themselves as “real”.
Therefore, our models of reality refer to arbitrarily long-lived versions of each person. …
Therefore I have reason to anticipate never dying.
Therefore I have reason to act as if I will never die.
Oh, I should have been more explicit: I think there’s a big logical leap between steps 3 and 4 of the rewritten argument, as indicated by the ellipsis. (Why is our models of reality referring to arbitrarily long-lived versions of each person a reason to act as if I will never die?) It’s far from clear that this gap can be bridged. That’s why I said the argument evaporates.
I think if you fully taboo the concepts of “personal identity” and “existence”, the argument evaporates. Before tabooing, your argument looks like this:
Alternate universes, containing persons psychologically continuous with me, exist.
Persons psychologically continuous with me are me.
Therefore I am immortal.
Therefore I should anticipate never dying.
Therefore I should make plans that rely on never dying.
On the face of it, it seems sound. But after tabooing, we’re left with something like:
Our best, most parsimonious models of reality refer to unseen alternate universes and unseen persons psychologically continuous with each other.
In such models, alternate futures of a person-moment have no principled way of distinguishing one of themselves as “real”.
Therefore, our models of reality refer to arbitrarily long-lived versions of each person.
…
Therefore I have reason to anticipate never dying.
Therefore I have reason to act as if I will never die.
Edit: My #4 and #5 were nonsense.
I either don’t understand your re-write or don’t understand how it dissolves the argument.
Oh, I should have been more explicit: I think there’s a big logical leap between steps 3 and 4 of the rewritten argument, as indicated by the ellipsis. (Why is our models of reality referring to arbitrarily long-lived versions of each person a reason to act as if I will never die?) It’s far from clear that this gap can be bridged. That’s why I said the argument evaporates.