On 10 November 2003 David Barrett-Lennard wrote:

"...It seems to me that the concept of identity is not fundamental to physics. It's useful for classification purposes as long as one doesn't stretch it too far and expose its lack of precision..."

David, I used to puzzle over the definition of this word also, but enlightenment did not come until I realised that granting it the status of a basic and precise concept was the cause of all the philosophical mischief. The "problem of personal identity" mostly goes away if we admit that there is no objective, yes-or-no answer to the question of whether two SAS's are the same person, and instead speak only of greater or lesser degrees of similarity. Of course, there are cases of common usage which most people would not question - that I am the same person today as I was yesterday, for example - but even here, I would argue that the identical/non-identical cutoff is (a) quantitative rather than qualitative, and (b) in the final analysis arbitrary. Certainly, the atoms comprising me-today are not all the same as those comprising me-yesterday, and if we consider me-last-year, none of the atoms may be the same. In fact, if we consider physical continuity the important factor in survival, the person that lived in my house last year is as thoroughly dead and gone as if he had been killed and cremated, and the ashes scattered to the four winds! Add to this all the other actually or logically possible adventures an individual could be subjected to - brain or body transplant, memory loss and dementia, uploading to a computer, melding or splitting of mind, destructive and non-destructive teleportation, resurrection in the far future or in Heaven, multiple or infinite versions in MWI - and it becomes clear that, if it has any meaning at all, "personal identity" must be a very slippery and plastic concept indeed. I feel much better for having got rid of mine!

Stathis Papaioannou
Melbourne, Australia
11 November 2003

-----Original Message-----
From: David Barrett-Lennard [mailto:[EMAIL PROTECTED]
Sent: Monday, 10 November 2003 6:40 PM
Subject: RE: Quantum accident survivor

I'm trying to define "identity"...

Let's write x~y if SAS's x and y (possibly in different universes) have the same identity. I propose that this relation must be reflexive, symmetric and transitive. This neatly partitions all SAS's into equivalence classes, and we have no ambiguity working out whether any two SAS's across the multi-verse have the same identity.

Consider an SAS x that splits into x1, x2 (in child universes under MWI). We assume x~x1 and x~x2. By symmetry and transitivity we deduce x1~x2. So this definition of identity is maintained across independent child universes.

This is at odds with the following concept of identity...

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 experience

It seems necessary to distinguish between a definition of identity and the set of memories within an SAS at a given moment.

Is it possible that over long periods of time, the environment can affect an SAS to such an extent that SAS's in different universe that are suppose to have the same identity actually have very little in common?

What happens if we "splice" two SAS's (including their memories)?

It seems to me that the concept of identity is not fundamental to physics. It's useful for classification purposes as long as one doesn't stretch it too far and expose its lack of precision.

This reminds me of the problem of defining the word "species". Although a useful concept for zoologists it is not well defined. For example there are cases where (animals in region) A can mate with B, B can mate with C, but A can't mate with C.

- David

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