Here’s my favorite philosophy of mind thought experiment that challenges pur view of personal identity or “self”. It takes a while to explain but is quite a mind fuck, so bear with me.
THE SETUP
It feels, to nearly everyone, on a gut level, that I am ‘me’ and you are ‘you’, and consciousnesses are distinct from each other and easy to separate. Moreover, the vast majority of people accept that you 1 minute from now is still the same “YOU” in a meaningful sense as YOU right now; that it makes sense for you right now to try to prevent harm to you one minute from now (even if acting purely selfishly – not altruistically trying to prevent harm coming to any being).
But imagine the following hypothetical scenario taking place far, far in the future, when our understanding of the brain, our ability to perform brain surgery, and our technology, in general, is drastically more advanced than today.
THE SURGERY
You’ve voluntarily elected to undergo an operation (for science!) where your brain will be slowly but completely replaced with a replica brain.
The steps of the surgery are as follows:
(1) you are temporarily put into a deep slumber (e.g., using something like general anesthesia) for a few hours.
(2) surgeons very carefully remove some tiny portion of your brain and then analyze that portion of the brain (e.g., using some sort of scanning device)
(3) based on this analysis, they then construct a replica brain piece (whether robotic or made of organic material, it doesn’t really matter) that will behave essentially just like the piece they just removed (e.g., maybe using some kind of 3d cell printer)
(4) they then insert this replica piece in the place where the original brain piece used to be and affix all of its connections to the surrounding brain tissue just as the original brain piece was affixed so that it will operate within the greater brain essentially just like that missing brain piece used to
(5) the original piece removed from the brain is then carefully preserved elsewhere, and its original position in the brain is carefully recorded
(6) finally, you are woken up, and you get to spend the day doing whatever you like before returning the next morning for further surgery
THE ARGUMENT
Now, after each time that steps 1-6 occur, you should feel almost exactly the same (minus any nuisances of having undergone brain surgery, but with this future technology, those are minor). After all, only a very small portion of your brain was removed each time, and that piece was replaced with a piece that was designed to be functionally identical. If the portion removed was small enough, and the replacement piece was a sufficiently accurate replica, you shouldn’t even be able to tell that you’ve changed at all.
What’s more, you remember your life before surgery, you remember being put to sleep on the lab table, and you remember waking up. You have no more gap in your life than you would after a sleep where you don’t remember your dreams.
So it seems that after steps 1-6, you are still “you” in any meaningful sense (or if not EXACTLY you, you are still really, really close to you).
Okay, but suppose now that steps 1-6 are repeated many times (replacing one tiny brain region each morning and then spending the rest of each day living your normal life).
At some point, 50% of your brain has been replaced (whether measured by mass, volume, or “functionality,” it doesn’t matter). Are you still “you” in a meaningful sense?
In my experience, at this point in the thought experiment, most people will say that it’s definitely still you. With each tiny replacement, you couldn’t tell the difference, and as long as those replacements were each accurate enough, you shouldn’t even be able to tell the difference with 50% of your brain replaced. What’s more, there was never even any large gap in your consciousness.
THE TWIST
But here’s the twist: as you realized a few months into the experiment (well before 50% of your brain was swapped), the surgeons had actually lied to you originally. You thought that they had been analyzing each piece of your brain bit by bit as they removed it in order to make replicas of each piece.
What they had ACTUALLY done (while you were asleep that very first time on the operating table) is scan your entire brain right away, building a nearly perfect, functioning, living replica, which they keep in a vat. This functioning replica is able to think just like you, has all your memories, and even BELIEVES that it is you. The replica is so accurate that, if the replica was hooked up to a machine that lets it talk, a person who knows you really well wouldn’t be able to tell which of you it was talking to (other than due to the fact that the replica is disembodied – which of course would change its behavior, but your own behavior would change in an equivalent way if you were disembodied).
So when the surgeons were removing tiny portions of your brain, they were also removing the closest possible corresponding tiny portion of the replica brain (after putting it to sleep in the same manner that they put you to sleep) and simply swapping it with yours. So the replica got your tiny portion, and you got the replica’s corresponding (and incredibly close to equivalent) tiny portion.
This lie from the surgeons doesn’t much change what happened to your brain FUNCTIONALLY since, in either case, each tiny portion of your brain is replaced with a very similar tiny replica portion. However, since the replica brain and your brain are having different experiences each day, there are genuinely irreconcilable differences between the tiny parts of the two brains (e.g., your brain may have some new connections in that tiny region that the replica doesn’t). The surgeons simply do their best to always swap the tiny part from your brain with the closest, most identical portion of the replica brain, even though the match isn’t 100% perfect. By making the tiny portions sufficiently tiny, you should still notice very little difference upon waking after each surgery.
IS THE REPLICA YOU?
Now, most people agree that when the replica of your brain is first created (before any swapping occurs), it is just like you, but it is not actually YOU. One way to think about this is that you still control your original body, and you still experience the thoughts of your original brain, whereas the replica separately experiences its own thoughts. In fact, you didn’t at first even know that a replica was ever created, you just went on having your normal experiences, so it seems that the replica being created had no effect (initially) on your identity.
But now that we’re at the halfway point, with half of the portions of your brain swapped out into the replica and half of the replica brain swapped into yours, personal identity becomes genuinely confusing. Both copies are now half original and half replica. In fact, both brains have a consistent series of memories, with no weird gaps (beyond the sort that one has from sleeping at night). The replica remembers going to sleep that first time on the surgery table just like you do; the only difference (as far as it is concerned) is that when it woke up, it didn’t have a body. Each day you and the replica both underwent surgery, and after each surgery, you both woke up being very close to identical to what you were prior to surgery.
While I think at this point most people still think that the original brain that slowly had 50% swapped out is more YOU than the replica that slowly had 50% swapped in, it’s gotten a lot harder to explain WHY. Both brains now have an equal amount of original material, and both feel equal, like they are the original you.
THE FINALE
The thing is, the surgeons aren’t done at 50%. They decide to go all the way, continuing steps 1-6 until 100% of your brain has been swapped with the replica brain.
The really weird thing now is that the body walking around that you used to identify as YOU now has the replica brain in its head! And the vat where the surgeons keep the other brain has your original brain in it! In other words, the two brains are now just back to what they started at, with the only difference being that during the interim period, they were temporarily mixed up (continuing to gain experiences while mixed up), and “your” brain happened to end up in the vat whereas the replica ended up in your body.
Compare this whole thought experiment to the surgeons just (all in day one) removing your brain and putting it into the vat, then making a replica that they put into your body. The actual thought experiment is very similar to this, except that with the slow process of swapping a tiny portion each time, your two identities get more mingled (since those brain regions are all slightly changing each day as the two brains have experienced, one in a body, one in a vat).
At this point, people tend to be much more confused, but I think most would say that the brain in the vat is now the real YOU. After all, it is essentially entirely your original brain.
WHAT WOULD A SELFISH BEING DO?
If this all sounds very abstract asking “which one is really YOU?”, then how about asking this more concrete question: now that you know the full details of the experiment, if one of the two brains (either the one in your body or the one in the vat) is going to be tortured in the future (after all the surgeries are fully complete) and you get to pick which of the two brains it happens to (you pick prior to the experiment beginning, of course), which one do you choose to be tortured (assuming here that you’re a selfish person that doesn’t want to experience torture)? You can also ask the same question (again, choosing before the experiment begins), assuming that the torture occurs after 50% of your brain is swapped with the replica (instead of 100%). In the 50% case, which brain would you choose to be tortured (the one in your body, or the one in the vat)?
Another way to think about what we mean by “YOU” in this thought experiment: suppose the brain that ends up in the vat and the brain that end up in the body both are properly hooked up to working eyes at the end of the experiment so that they can both see, and furthermore that when the experiment has totally finished the brain in the vat will get to see a picture of Tom Hanks, whereas the brain in the body will get to see a picture of Bob Saget if you (before the experiment has started) are asked “Later when the experiment is over will you see a picture of Tom Hanks or a picture of Bob Saget?” what would you answer? If you say “Tom Hanks,” then you expect to have the experiences of the brain that ends up in the vat, whereas if you say “Bob Saget,” then you expect to have the experiences of the brain that ends up in the body. Note that if, for someone reason, this question lacks meaning for you, then note that outside of weird philosophy of mind thought experiments, this sort of question is not only meaningful but has an obvious answer. For instance, if you knew someone was going to show me a picture of Bob Saget tomorrow and show you a picture of Tom Hanks tomorrow and someone asked which picture you expected to see tomorrow, you would say “Tom Hanks” – duh!
THE AFTERMATH
How can this scenario be interpreted?
(A) We could say that the brain in the body started out as YOU, and at some discrete point, things flipped so that YOU end up in the vat. But what seems to make no sense about that is where to draw the line on that flip. The only way this could happen is if there was exactly one surgery (which, remember, only after swaps out a tiny, functionally nearly identical brain region) which caused the brain in the body to flip to no longer being you and the brain in the vat to being fully you. If there was some special “soul” molecule in your brain which determined what was you, then this would make sense (since the moment that soul molecule was transferred, the brain in the vat would become you), but nobody seems to believe in such a soul molecule (neither scientists nor believers in souls).
(B) We could say that in the end, neither of the two brains are YOU in a meaningful sense. This seems pretty weird since, after the first small surgery, it seems nearly certain that the brain in your body is YOU in a meaningful sense (or very, very nearly you), so what operation prevents the being in the vat (which has all your original brain matter essentially, and has all your memories, and has the daily experience of being you daily) from being you? What exactly about this whole procedure fully eroded your identity?
(C) We could say that both brains are about equally much you (say, each close to 50% you). This seems weird because at which point did you split into two? It also seems weird because the brain in the vat is actually made of essentially all of your original brain material, so how did this slow process of taking piece by piece somehow make the replica brain have equal claim to being you? After all, if all the surgeons had done is take your brain out and move it to the vat immediately, we’d all agree that the vat brain was you, not the replica.
(D) We could say that BOTH brains end up as fully you. This seems even weirder because it’s not like what one of those brains thinks will causally change the behavior of the other. And the brain in the vat definitely doesn’t control your original body. So they can’t both be you in the sense of both being one indistinguishable consciousness. But maybe they could both be you in the sense of having equally strong claims to being you?
(E) We could say that this thought experiment is physically impossible. In order to work (as a thought experiment), it doesn’t actually matter how incredibly hard or unlikely this scenario is (the point is simply asking how we should interpret it if it WERE to occur). But if something about the thought experiment actually violates the laws of physics, then we can dismiss it, saying that it couldn’t take place in our universe, so the question being asked is not actually a question about our universe. But what aspect of the thought experiment violates the laws of physics?
(F) We could argue that although it doesn’t violate the laws of physics, the only way to do it without violating those laws would be to have the tiny swapped brain regions be a lot more different from each other than I imply. In other words, doing swaps of tiny brain regions is not impossible, but it’s impossible to make them extremely similar (e.g., because the brain is changing too fast each day, or because of error in such a procedure that is impossible – it’s directly implied by physical laws). This conclusion might push us a bit towards (B), thinking that neither brain is really you (they get too messed up in the process, so both drift from you in a meaningful sense).
(G) You’re still [the brain in] the body. You stay in the body (fully and determinately) with every small exchange, so you stay in the body after the full exchange process is completed.
(H) We are so deeply confused about identity that when we say it is or isn’t YOU, we don’t realize that we aren’t actually saying anything sensible/meaningful; we only think we are.
(I) We could say that YOU started out in the body, then for a while you were mingled with this other being merging your YOUNESS, but, by the end, you have separated again with YOU ending up in the vat (for much the same reason that we would say you are in the vat if your brain had originally just been cut out immediately and put in the vat – only in this case it was done bit by bit instead of all at once causing a temporary period of being merged).
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