A man came up looking for a solution to live forever. Arguably, that should be totally doable. Since humanity in the 21st century believes that human nature is but the result of brain activity, all it takes to live forever is keeping a brain alive forever, a challenging feat to be sure, but nothing prohibited by the laws of physics. As a prototypical man of the Technology Information era, his understanding was that we, humans, are not that different from computers: we have a brain, which is equivalent to a computer's hardware, and we are the software that tells the hardware how to do stuff. Therefore, in his view, what makes us us is the code, the software the runs the brain. The information processing that goes on the brain supposedly (magically) generates consciousness. He also understood that the cerebral program was coded into the brain through neuronal plasticity, meaning, the software was actually encoded via physical modifications of the actual hardware's interconnections and not as high-level instructions executed by an invariable structure.
As a neuroscientist, I was kind of used to keeping brain slices artificially alive. The technique of Brain Slices, invented in the 20th century, was quite useful for drug development and intraneuronal studies. Also, having worked with brain mapping of candidate patients to epilepsy and brain tumor surgery, I had a general understanding of the different areas of the brain and their functions. Better yet, I was aware of a "new technology", which would allow me to keep this man's brain alive artificially and indefinitely. I would monitor the activity of all of his neurons and, as they started malfunctioning, I'd simply replace the original neurons with synthetic ones, which were based on this "new technology".
And here starts our thought experiment.
For decades, I followed that man, continuously mapping his brain activity and the health of every neuron. Every time a neuron got sick, I'd just replace it with a new, artificial one, which was rated for at least 1,000 years. Meanwhile, other doctors and scientists would make sure his other organs were functioning properly as well, so that the brain would never run out of a body for it to play with.
By his 200th anniversary, my client's artificial brain was performing wonderfully and this man was quite happy, feeling at the top of his mental and cerebral game. Fortunately, his body was equally flawless and he was living a very accomplished life. (In order to care for him for such a long time, I found another scientist to use the same technique on my own brain as well).
A few decades later, the man started feeling a little bored. He had lived all the experiences available to man and the technology wasn't keeping up to provide him with new and more exciting experiences. He asked me if I would put him to sleep for a few decades, so that the world would have the time to develop some more interesting stuff for him to do. "No problem," I said. We agreed that I would take good care of his brain while he was away. Sweet dreams.
When he returned, though, a few decades later, he thought he needed a new body, more in line with the other bodies of the time. Perhaps a body that could fly or submerge and breath under water. Given body-science developments, I proposed to simply transfer his brain to a brand new body. He agreed. Since all he was was his brain, a physical machine, we also reasoned he wouldn't really need to keep using his old, outdated brain. It'd make more sense to simply transfer the old brain into a new one that would be based on newer technologies. Also, the new brain would allow for some added enhancements, which would include an universal translator and other knowledge that he would know, but would never need to study to learn.
As the computers that our brains are, we knew that, to transfer someone or someone's brain (which actually are the same thing, right?) into a new brain, it's not enough to simply replicate the hardware. Most importantly, we need the installed software to be the same and, particularly, the system "state" to remain the same in the new brain to guarantee my client would continue to be exactly him. To capture the exact state, we would need to "hibernate" the man, which, in biology, is accomplished by freezing. And so we did.
We froze the man and his brain and started the process of transferring his old brain into a new brain. At this point in time, the process is quite simple. It basically consists of 3D-scanning the old brain and 3D-printing an exact copy of it to nano-scale detail. Unfortunately, there still was no way for us to make a brain copy to the quantum-scale level, because of all those fuzzy and the ever fluctuating states, which are not readily copyable. Nevertheless, our technology was a marvel of science and, since what makes us us are the neural networks in the brain, there's no need to worry about that quantum mess.
The process goes on flawlessly and, after a few hours, we have our client back, in his new, unfrozen superbody and his new brain. He woke up after unfreezing just as he usually does after a good night of sleep, which was expected. As previously agreed, we would exhaustively test the new brain in the new body to make sure the process concluded flawlessly and would not need to be repeated.
The final result was perfect! He was soon eager to see his friends and family members, who were unaware of the update just to allow for a blind validation that the man continued to be exactly him. That night, there was a dinner party to gather many of his more direct acquaintances. Our team followed him up close while discretely observing and measuring every interaction, every behavior and every human interaction during the party. By the end of the night, there was no doubt: the software and state transfer to the new brain was a real success. The client was very happy and nobody noticed a thing. He continued to be fully him. We did not attempt to try the new brain enhancements, like the translator, because there were no foreign guests at the party to practice with. By just noticing he was able to speak other languages was already very exciting for him at that point (let alone the new ability to fly and breath under water!).
Next day, he came up to me to ask about the rest of the plan. Gosh, I wished that part of the transfer had failed, but he remembered... In any case, per contract, I was legally bound to unfreeze the original body, since the original client wanted to be sure it was really him in the new body. That was way out of protocol, which should have precluded me from waking up the old body, but he managed to push it on me anyway considering he was such a great sponsor of my research... The man in the new body was happy enough and wasn't really interested in waking up the old body, but he also remembered the agreement and the contract. And I had given my word, damn it...
We proceeded as originally agreed and unfroze the man in his original body with his original brain (as a matter of fact, at that point, his brain was anything but original, since I had already replaced most of the neurons with synthetic replacements). The man in the new body couldn't care less and went flying.
"Welcome back!", I greeted him in his old body. "I have great news! Your brain transfer was a huge success!".
"Oh, good morning doc, I had the most amazing dreams while I was away. How long has it been?"
Me, being a "great" scientist and fully ignoring his talk about dreams: "It's been just about 48 hours. We were able to verify that you in your new body and brain are just the same old you. You're good to go for at least 2,000 more years now."
"That's interesting. Can I see him?"
"Well, that'd be totally against protocol. You should just trust me you're doing great out there."
"I'm sorry, but I can't trust that. If he's that and I'm here, that means I'm not there and I'm not him."
"Well, you're kinda right and wrong here. We do have two instances of the same program running in two different brains (they're actually equal, but distinct brains), but both instances are indeed you. Who else could that other instance be if not you, right?"
"I don't know, doc. All I know is that that one isn't me. As soon as you shut me down, I'll be gone for good and that was definitely not part of the plan."
"Listen, I can assure you: you are living, you are well and you are having a great time! You're just no longer using this old body and brain."
"Can he really fly?"
"Man, I wish I could that"
"But you are!"
"But it doesn't feel like it."
"Don't worry. As soon as we shut down this body, you'll be totally fine. Unfortunately, you won't be able to feel it while you continue to run in this body. That's why we need to shut you down."
"I'm sorry, doc, but I'm having second thoughts now. I don't trust that man to really be me."
"Well, he surely is you. His brain is a perfect copy of yours, sharing exactly the same state. Whatever consciousness that brain is generating, it is you! That brain is only capable of generating the very same consciousness that your old brain does, the one that I'm talking to right now."
"Well, if you're talking to me now and not to him, we are definitely not sharing the same consciousness. We're just two equal, but independent consciousness. I really don't think this is going to work. I want my own brain to be transferred to the flying body."
"But that's what we did! It is your brain that's there."
"It's not! That one is a copy!"
"But if you think scientifically, you should know your current brain is no longer original either. I've already replaced almost all its original neurons with new ones. Your current brain in this body is essentially a copy of your old brain, just like the brain in the new body is just a copy of this brain.
"I'm confused, this is the first time you make a copy of my brain."
"Actually, this was the first time I made a separate copy of your brain, but your brain is no longer your original brain. That has been the case for a long time now."
"That's it! That separate copy is not me. The incremental improvements you made were still me, but that separate thing is someone else. I want this brain to continue to be upgraded, instead of getting a new one."
"You realize there's no real difference, right? Every time I did an upgrade in the past, I hibernated you. While you were hibernating, your brain was literally dead. There was no neural activity at all! It remained a dead brain while I was doing the improvements, which is a requirement to preserve the "state." As you can see, upgrading brains using our current technology requires me to deactivate the brain to preserve its state to then reactivate it in the same state as before. That's how you continue to be exactly the same as before after the procedure."
"You mean you killed me every time before bringing me back?"
"Not quite. If I had killed you, you wouldn't be here, right?"
"You are your brain activity. As long as your brain is working, you are alive and you are you. The only difference now is that you insisted to be brought back in your old body and brain instead of just moving on with your new body and brain. If you continue to be using the old brain, we'll have a big problem. Your two brains will continue to develop independently and this you and your other you will start to diverge and will no longer be the same. Also, your wife has made it very clear that she loves you, but she can't handle two of you."
"Typical... Doc, let's move this brain to the new body."
"But it's the same thing! This brain will hibernate and will wake up in another body. The brain that is already there is structurally, connectively and logically the same as yours now, but it has better materials and will last much longer. While the brains are hibernating, they're just resting matter. They're scientifically indistinguishable from one another. If I didn't label them, I wouldn't even know which one is which, no matter how much I tested them. They were built to be equal!"
"OK, doc, enough arguing. You will put this brain in the old body and that's it."
"Well, then what do I tell the other you?"
"You figure it out. It's your grant at stake."
About 48 hours later...
"Welcome to your new body! You now have flying and submerging powers!"
"Hey doc, good to see you again! I had the most amazing dreams while I was gone!"
"Glad to hear! I want to hear all about it, one day..."