Personal Identity Across Possible Worlds

Does your personal identity extend across possible worlds? It seems that typically metaphysicians take it to be the case that a person can have a counterpart in a different possible world - for example, I, Sofia at the actual world (@), might have a counterpart in possible world w, who is exactly like me in all respects except she didn't start this blog post. Is that Sofia also me?

The answer I will give you is the complex, "yes and no". Some metaphysicians will say "yes" without batting an eyelash, while others will growl, frown, and give you the evil eye for even asking.

The following thought experiment is meant to help disentangle these fundamental disagreements at the most hair-splitting level (in the tradition of the best analytic philosophy, which is typically anal-retentive like nobody's business).

Imagine an animal cell in the midst of Prophase. It is living its typical animal cell life, absorbing nutrients through its cell membrane, maintaining homeostasis, and getting fatter. Simultaneously, its DNA copying proteins are hard at work, making a nucleotide-by-nucleotide replica of its existing genetic data.

Now imagine this cell - let's call him Sparky, for fun - doing exactly the same thing in exactly the same ways in exactly the same physical location, but in different possible worlds. So we have Sparky at w1, Sparky at w2, Sparky at w3, and so on, and all of these Sparkies are undergoing DNA replication. Before someone protests that I'm begging the question about Sparky's identity across worlds already, I will make a concession and call Sparky at w1 Sparky1, and Sparky at w2 Sparky2, and so on. It's like the metaphysical version of terrible chatroom names. They could be different individuals, or they could be the same one; but because they're in the same place at the same time doing the same thing, I've given them similar names to make them easier to refer to when we're comparing their relative fates.

So, as cells do, all of these Sparkies will go on to split into two different daughter cells, neither of which will be the original Sparky, but each of which will have the same or roughly the same (depending on how replication goes in the particular world) genetic information.

To make this easier for ourselves, let's establish an (equivalent) point of reference in each world relative to each Sparky, and call the resulting daughter cells either Lefty or Righty, depending on their relative location to us. Thus, we'll have Lefty1 and Righty1 at w1, Lefty2 and Righty2 at w2, and so on, and each LeftyN and RightyN will be the daughter cells that result from the split of SparkyN at wN.

Now, no claims about identity across possible worlds have been made yet. I have simply named everyone in a way that will make everybody easier to pick out.

[...to be continued]