A person is placed under general anesthesia and - using some form of unknown phlebotenum - an exact replica is made. The replica has all of the original’s memories, personality traits, and is exactly like the original in every physical way. The replica is also unconscious. Two blindfolded people enter the room and wheel away the original and replica (or at least the ones that somebody was randomly told to tell them might be the original and the replica).
The two are placed in separate nondescript rooms and they both awaken completely alone. Then somebody enters both rooms and tells the occupant that the process was a success and that the replica is waiting in another room. But there's a problem: the replica believes that it is the original, and the only one it will listen to is its twin.
It is up to the original to convince it’s replica that it is, in fact, not the original. So the original and the replica are taken to a third room, each one given the exact same scenario.
Moon (2009)
Is it possible for the two to determine, between themselves, which one is the original and which is the replica?
If it is not, then is there still an original consciousness? From an objective view, an original still exists; it was required in order to make the replica. From a subjective view as well, though each of the two believes that they are the original. There must be one original and one replica.
Could the two be sharing a consciousness between them? I posit negative, because the same consciousness cannot exist in two places at once; the consciousness is a result of physical processes, and there are two distinct physical substrates in existence. If an idea occurs to one person it won’t necessarily be known to the other. So while the two are physically identical and sharing identical experiences, there are two distinct and unique entities, if only for occupying different space-time coordinates.
While it may not be possible for the two to determine which one is the original, the fact is that one definitely is.
But does it make a difference? To the person who thinks they are the original, you bloody well bet it does!
Now imagine the same scenario except, when the blindfolded people enter, they push one of the bodies into the nondescript room, and the other into an incinerator, again based on a random and blind assignment of identity. The remaining individual is awakened and told that the process was a failure and that no replica was created. Does the consciousness that began this little thought experiment still exist?
There is no way for the person who remains to know, and perhaps it would never occur to the person to think that it wasn’t the original. From the resulting person’s point of view, it is the original. But there is a 50% probability that the person who walks out of that building is not the person who went in; that the original consciousness is no more; has ceased to be.
When it comes to brain replication as a form of immortality, does this matter? The person who results is the same in every way as the one who underwent the procedure, and never can know whether they are original or replica. Whether the original or the replica is destroyed, the person who leaves is none the wiser. But one of them definitely died. When the two were in the same room, trying to determine who the original was, they both had the same experience, yet both were distinct from the other; for all intents and purposes, two people. Does the immediate destruction of one before both become conscious and diverge their personalities change the fact that two still existed for however short a time?
Let’s place your consciousness in the brain of the original going into the procedure. You're both in the room, awake, and meeting the other for the first time. From your point of view, you are the original and the other is a replica. You are one consciousness and the other is a copy. You think alike, but you do not know what the other is thinking. The other has thoughts sovereign to itself, and, from their point of view, you are a copy.
How would you react?
Is there any situation in which such a procedure is desirable?
What if only one person would survive? (No guarantee it's you.)
Is that better than both surviving? Worse?