Therefore, one could argue that the idea of an "individual" existing through time has no objective basis to begin with, and the decision to identify entities that exist in different instants of time as the same "individual" can't be other than a subjective whim.
Evolution may have reasons for making us think this, but how would you get that the identification of an individual existing through time is subjective? You can quite clearly recognize that there is a being of approximately the same composition and configuration in the same location from one moment to the next.
Even (and especially) with the Mach/Barbour view that time as a fundamental coordinate doesn't exist, you can still identify a persistent individual in that it is the only one with nearly-identical memories to another one at the nearest location in the (indistinguishable-particle based) configuration space. (Barbour calls this the "Machian distinguished simplifier" or "fundamental distance", and it matches our non-subjective measures of time.)
ETA: See Vladimir_M's response below; I had misread his comment, thereby criticizing a position he didn't take. I'll leave the above unchanged because of its discussion of fundamental distance as a related metric.
SilasBarta:
You can quite clearly recognize that there is a being of approximately the same composition and configuration in the same location from one moment to the next.
That's why I wrote that "the concept [of personal identity] is more or less coherent assuming the traditional biological constraints on human life." It falls apart when we start considering various transhuman scenarios where our basic intuitions no longer hold, and various intuition pump arguments provide conflicting results.
Arguably, some of the standard arguments that come...
It’s the year 2045, and Dr. Evil and the Singularity Institute have been in a long and grueling race to be the first to achieve machine intelligence, thereby controlling the course of the Singularity and the fate of the universe. Unfortunately for Dr. Evil, SIAI is ahead in the game. Its Friendly AI is undergoing final testing, and Coherent Extrapolated Volition is scheduled to begin in a week. Dr. Evil learns of this news, but there’s not much he can do, or so it seems. He has succeeded in developing brain scanning and emulation technology, but the emulation speed is still way too slow to be competitive.
There is no way to catch up with SIAI's superior technology in time, but Dr. Evil suddenly realizes that maybe he doesn’t have to. CEV is supposed to give equal weighting to all of humanity, and surely uploads count as human. If he had enough storage space, he could simply upload himself, and then make a trillion copies of the upload. The rest of humanity would end up with less than 1% weight in CEV. Not perfect, but he could live with that. Unfortunately he only has enough storage for a few hundred uploads. What to do…
Ah ha, compression! A trillion identical copies of an object would compress down to be only a little bit larger than one copy. But would CEV count compressed identical copies to be separate individuals? Maybe, maybe not. To be sure, Dr. Evil gives each copy a unique experience before adding it to the giant compressed archive. Since they still share almost all of the same information, a trillion copies, after compression, just manages to fit inside the available space.
Now Dr. Evil sits back and relaxes. Come next week, the Singularity Institute and rest of humanity are in for a rather rude surprise!