Are Multiverse Possible In The Real World?

[jayem]

And if our own universe exists as a mathematical object somewhere in that transcendental realm of pure math... then there's no particular reason to suppose that it exists in any other way, is there? Like, the existence of the universe is accounted for. So we're good on that front.

While it works it can go in the box. It doesn't of course get to claim precedence (unless we are explicitly talking about it, in which case it can).

I don't know what "privileged position" means in this context (on account o' bein' a simple self-taught philosopher without lots of fancy formal schoolin').

Basically that I'm me, so I know that me and Jay (despite similar names) aren't sockpuppets (to a deeper level than just relying on the boards abilities to detect misuse). You don't know that because you don't have that privilege of being me. You may sincerely propose it, I know that you are wrong, however much evidence you bring (unless it involves serious dual personalities).

Your you, I don't know if you and Jay are sockpuppets, you do because you are you. I don't have that privilege. I have to go on lesser evidence (in this case probably not)

In the Harry Potter case there are good reasons to assume she's making it up, however were that to be in question then we'd have to be extremely careful about assuming that as part of our argument. If we were wrong and Harry's reading this, he would be quite right to laugh at our wrongness. If we were wrong the other way J.K.R would be (possibly not quite as rightly) laughing.

My question is whether Harry would be right to conclude he exists if he could do that.
There are two possibilities here:

1. If Harry Potter were to exist, think, and argue "Cogito, ergo sum", then that argument would be correct and Harry's existence would be proven.
In that case, it seems to me that Harry's position would be symmetrical to yours.

No problem there, (of course that's the important if in there). In fact that of course works with a context dependent 'exists'. Also it's only proven to Harry in both cases.
1. If Harry Potter were to exist relative to him, think, and argue "Cogito, ergo sum", then that argument would be correct and Harry's existence would be proven to him relative to him (of course in this case it says nothing about his existence relative to me...).

2. If Harry Potter were to exist, think, and argue "Cogito, ergo sum", then that argument wouldn't be correct because that argument is only correct when real thinking beings actually make it, and Harry doesn't actually exist.

Um...I think somethings got lost in writing.

(If (you can not-exist and you think) then (if I think therefore I exist) is a false statement) Is a true statement (effectively saying Descarte is wrong if he is wrong)
If you are not-existing then you can't think. Is basically the contra-positive of Descarte (which is of course an assumption)
If (you can exist and not-exist) then XYZ is a true statement, but one for which the premises will always be false so not a particularly helpful one.

[Devils_Advocate]

I guess that I wasn't quite careful enough with my grammar. The question that I was getting at was: If a mind that doesn't actually exist were to [exist, think, and argue "I think, therefore I am"], would that argument prove that mind's existence?

If yes, then maybe that instance of that argument technically still "doesn't" work, but once we've established that the difference between "does" and "would" is relative, doesn't it seem like less of a thing to make a big deal out of?

If Harry Potter's situation would be in all relevant ways the same as my situation would be, then neither of us would have any more justification than the other to conclude that anything about only that one's situation is, save in a purely relative sense.

Basically that I'm me, so I know that me and Jay (despite similar names) aren't sockpuppets (to a deeper level than just relying on the boards abilities to detect misuse). You don't know that because you don't have that privilege of being me. You may sincerely propose it, I know that you are wrong, however much evidence you bring (unless it involves serious dual personalities).

Your you, I don't know if you and Jay are sockpuppets, you do because you are you. I don't have that privilege. I have to go on lesser evidence (in this case probably not)

I don't follow. The ideal secret agent is not even aware that it is a secret agent. If I wanted to keep secret that someone was an alternate persona of mine, it wouldn't know itself to be an alternate persona. That is to say, I would imagine what someone who didn't think it was a sock puppet would type, and type that. Because any way in which a sock puppet's posting was affected by knowledge that it was a sock puppet would be a potential "tell".

How could I possibly know myself not to be an imaginary entity specifically constructed to have experiences and beliefs that differ from those of a real person in no discernible way? Because my subjective experiences are real? Assumes facts not in evidence, bucko! Obviously I expect that an ideal fictional secret agent would be imagined to naively regard its subjective experiences as real, as that's the ordinary and thus inconspicuous option. So, if anything, my best evidence that I'm not a sockpuppet seems to be that I take the possibility of being a sockpuppet seriously!

(Frankly, I think that there are better ways to decide whether an account is legitimate that don't have anything to with whether it's used by a "real person". "Reality" and "personhood" aren't particularly suited to the task and seem far more vague than necessary. Besides, I suspect that non-persons and the non-real are unfairly marginalized groups for most potential definitions of each.)

While it works it can go in the box. It doesn't of course get to claim precedence (unless we are explicitly talking about it, in which case it can).

It's significant because it doesn't seem to have a lot of competition. Perhaps I'm simply ignorant on the matter, but I'm not aware of a lot of theories of how or why things exist in general. What else is there? Platonism? But we're basically talking about a sort of formalized Platonism here, not something different altogether. Is there an altogether different option?

Also relevant to this thread is that the MUH posits the biggest multiverse. Hence why it is also known as Ultimate Ensemble Theory. It has all of the universes. All of them.

[jayem]

I guess that I wasn't quite careful enough with my grammar. The question that I was getting at was: If a mind that doesn't actually exist were to [exist, think, and argue "I think, therefore I am"], would that argument prove that mind's existence?

In a similar way to "If my aunt were a man, she'd have testicles" tells you anything. Giving that you are assuming the contradiction in the premise, you need to be very careful what you say and lay it out (probably worth using 'proof by contradiction' as a template).

How could I possibly know myself not to be an imaginary entity specifically constructed to have experiences and beliefs that differ from those of a real person in no discernible way? Because my subjective experiences are real? Assumes facts not in evidence, bucko! Obviously I expect that an ideal fictional secret agent would be imagined to naively regard its subjective experiences as real, as that's the ordinary and thus inconspicuous option. So, if anything, my best evidence that I'm not a sockpuppet seems to be that I take the possibility of being a sockpuppet seriously!

Or "Dubito, Cogito Ergo Sum".
This is what I was saying about it this being a separate thread.
We can create total doubt about our existence, but at that point it applies across the board. If we have it unmarked on the same thread as this then it's a recipe for applying it selectively and running the goalposts back and forth.

Lets make sure we're at least a bit consistent (or at least picking outinteresting inconsistencies). Lets talk about total doubt regardless of whether the universe (noniverse?) we mightn't live in is part of a bigger structure or not (at least until it makes a difference). Lets talk about the distinction (or not) between isolated universes, hypothetical universes, and interacting universes with at least some form of "I (and a reasonable number of obvious things) exist" assumption. When either of these are at an interesting point, then lets play with merging the two questions together.
We might come up to a good argument why (a particular view) works or doesn't that works that doesn't depend on everything being an illusion (including the illusion recursively). We've already (not the first to do so) ruled out the boringly small multiverse (at least without there being some reason why Jetpack Hitler is special).

Also relevant to this thread is that the MUH posits the biggest multiverse. Hence why it is also known as Ultimate Ensemble Theory. It has all of the universes. All of them.

Yeah, I was aware this messed up some things (albeit I think you could in theory create a hypothetical universe not in it, that wouldn't be an easy task and would be a recipe for sophistry, perhaps one with a matched spin fermions). I've been treating Harry Potter like he's purely made up by JKR but there is at least a hypothetical universe where "avada kevada" just happened to correlate with spontaneous heart failure (up to now) and presumably the multiverse would at least contain that. I'd say that's technically the opposite problem, "is there now no such thing as an 'imaginary universe'". Until we rule out one or the other, lets not assume it.

[Devils_Advocate]

Note that, from my semantic deconstruction perspective, maximum skepticism is basically a matter of doubting whether any word has meaning. Although really it's about questioning whether any concept is really a concept of anything, and questioning what that means. (E.g. A belief that your concepts correspond to some sort of external reality is itself simply a relationship in your mind between your concept of concepts, your concept of correspondence, and your concept of external reality; so where and how does the supposed "external reality itself" come in?) But it seems likely that words refer to things by way of concepts to the extent that they refer at all, so doing one is pretty close to doing the other.

I see the subjects of ontology, epistemology, and semantics as heavily interrelated. Knowledge is (a type of) true belief, a belief is an idea that something is true, a true statement means something real, etc.

If you start with the unexamined implicit assumption that your concept of existence is even coherent and even potentially corresponds to anything, then you're already taking a lot for granted. Perhaps I myself am still making an infinite tower of assumptions that I don't even know I'm making, yet I... doubt that Descartes ever came close to out-doubting me?



YEEEAAAHHH!

This is what I was saying about it this being a separate thread.

Okay, I think I get it now. If I understand correctly, you're complaining that I've been separately exploring the implications of various different assumptions without making explicit which assumptions I'm working with, and that switching back and forth like that makes it difficult to tell what assumptions I'm working with in any given time.

It's understandable that that could be confusing, so I'll try to be clearer about it. Perhaps it would be a good idea for you to be clearer about your own assumptions as well? (Not that hypocrisy renders any criticism incorrect, mind you. The kettle isn't any less black because the pot's saying so. So before replying "NO U", one should bear in mind that the NO does not follow from the U, as it were.)

Giving that you are assuming the contradiction in the premise

I don't think that I assumed a contradiction at any point. Let me try to lay things out in a bit more detail, and you can tell me if there's a step that you think contains a mistake.

1. Either every logically possible mind actually exists, or some logically possible minds do not actually exist.
2. If every logically possible mind actually exists, then there are no non-actual logically possible minds; QED.
3. If some logically possible minds do not actually exist, there nevertheless are internally consistent hypothetical scenarios in which they would exist. That's what it means for them to be logically possible.
4. Let us assume, for the sake of argument, that in at least one internally consistent hypothetical scenario, one of those non-actual logically possible minds would think and argue "I think, therefore I am".
5. Either that logical possibility is sufficient for that logically possible mind to actually exist, or it is not.
6. If it is sufficient, then then QED.
7. If it is not sufficient, then it is logically possible for a mind to think and argue "I think, therefore I am" without actually existing.
8. If it is logically possible for a mind to think and argue "I think, therefore I am" without actually existing, then a mind thinking and arguing "I think, therefore I am" does not logically prove that that mind actually exists!

See, the "trick", if you will, is that I'm exploring the idea that the actual is absolute. I.e. that the actual is something "above and beyond" the merely logically possible. And given that, it makes no sense to think of anything actually being the case in some internally consistent hypothetical scenarios but not others; either something actually is the case or it actually isn't! If the actual encompasses some logical possibilities but not others, then there's no contradiction in something not actual being logically possible. Something not actual being actual is a contradiction, but I never posited that.

You can have absolute actuality or "I think, therefore I am" can be sufficient to establish your actuality, but for the two to exist side by side... well, at the very least all logically possible minds that argue "I think, therefore I am" would have to actually exist.

Or "Dubito, Cogito Ergo Sum".

That doesn't prove anything! I could be an imperfect fictional secret agent designed to illustrate a philosophical point!

Honestly, the main reason that I can't justify concluding that anything is actually anything in whatever absolute sense you intend is that I don't know what that sense is. How can I conclude that anything is actual if I don't know what that means?

Do you know what you mean? If so, what is it, in a "path out of the walled garden" sense? And if you don't, how can you reasonably conclude that anything is actual? Your subjective experiences are just a specific case of that general issue.

[jayem]

That doesn't prove anything! I could be an imperfect fictional secret agent designed to illustrate a philosophical point!

I was struck by how much the last sentence matched the Descarte bit about doubt. It struck me as an amusing pseudo co-incidence.
____________
And yes my communcation also sucks, which is partly why I don't want to have to try and answer in both questions at once.
Don't get me wrong, it's an interesting discussion, relevant. In fact it's an interesting use of the way I'd want there to be a word to be useful.

Case 1 Brain in Vat Theory (and beyond):
The toothbrush I'm holding clearly 'hypothetically exists' so we can run your argument on it
I'd argue that (in this Brain in Vat case) the toothbrush doesn't 'exist'
Case 2 What we see is what we get:
In this case the toothbrush of course exists
A pink elephant that I am falsely asserting is in my room does not
Case3 The Small-connected Multiverse case (where at least one of the universes is one where there is in fact a pink elephant in my bedroom) and beyond:
In this case the "universe where there is a pink elephant" exists.
The pink elephant exists in that universe
The pink elephant in my bedroom in 'my' universe does not exist


They are in fact, rather circular. If you actually asked what the brain in the vat family of cases was...
It does however additionally show that in case 3, 'exists' is being stretched. If we used Case 3 regularly I think I'd want 2 words. If we used Case 1 regularly I'd want two words for non-existence.

Spoiler: additional thought 1

Show

A while ago I considered the difference between the things that Exist and what I called Interactables
I jumped to the conclusion that Interactables were (probably) a subset of Existing things (with the two being identical, if and only if Case 2 was the truth, whole truth and nothing but the truth)
You (Devils Advocate) thought I had written it backward and I was baffled as to why.

If Case 1 were the case, then Existing things would indeed be a subset of Interactables. I.E there would be things we (falsely) detected existing that did not 'exist'.
My mind was not on "Brain in a vat" and I didn't really consider it. I did vaguely leave some space (Veti's thoughts on King Arthur, made it a bit clearer what was nagging me). I missed that.

On the other hand in Case 3 there would be potentially Existing things that are not Interactables (also in the case of the universe being significantly bigger than ever-visible-universe). You might of course want to use a different word to 'Exists', so long as it doesn't include anything else (so it can't be hypothetically-exists or non-exists without some other qualifier). But it would be fulfilling the same function as the way I'm using it here.

_________
I think the ropey step is between step 4 and 7.
In step 4, you've created a hypothetical example where you've assumed that the (previously assumed non-existing) object is real and drawn a conclusion
When you come back to step 7, you've taken it out of the example.
What you have, I think, is a disguised version of this (I'll have to come back to it, because it isn't quite as simple)

  Suppose(1) there is no such X that has this property
       Suppose(2) instead that there were some X
  Therefore in our supposition there is some X that has this property

Spoiler

Show

3. If some logically possible minds do not actually exist, there nevertheless are internally consistent hypothetical scenarios in which they would exist. That's what it means for them to be logically possible.
4. Let us assume, for the sake of argument, there is in at least one internally consistent hypothetical scenario (S1), one of those non-actual logically possible minds(M) would think and argue "I think, therefore I am".

So we're taking a logically-possible mind (M). By 3, this is a mind for which there is an the internally consistent scenario in which it exists (S2). I presume we are in said scenario (S1=S2). I.E so we are assuming we are in a hypothetical scenario where the mind exists.

(it then in that hypothetical scenario does some stuff, which may or may not prove it exists, the conclusions are only valid given the assumed premises, including M's existence)

7. If it is not sufficient, then it is logically possible for a mind to think and argue "I think, therefore I am" without actually existing.
We've not shown that a mind that doesn't exist, if it were instead to exist could ...

8. If it is logically possible for a mind to think and argue "I think, therefore I am" without actually existing, then a mind thinking and arguing "I think, therefore I am" does not logically prove that that mind actually exists!

I quite agree this is tautologous, but we've not shown the premise (7)

Of course that assumes S1=S2
Of course there is a hypothetical scenario(S3) where jayem(M1) can't speak
and a hypothetical scenario(S4) where jay(M2) can speak

4. Let us assume, for the sake of argument, there is at least one internally consistent hypothetical scenario (S3), in which one of those non-actual logically possible minds (M2) would say "I think, therefore I am".

I think goes nuts...

[Devils_Advocate]

I don't want to have to try and answer in both questions at once.

Which two questions were those?

Case 1 Brain in Vat Theory (and beyond):
The toothbrush I'm holding clearly 'hypothetically exists' so we can run your argument on it
I'd argue that (in this Brain in Vat case) the toothbrush doesn't 'exist'

If one asked someone what fire was and he gave an answer that involved phlogiston, would the non-existence of phlogiston mean that by "fire" he didn't refer to anything real? It seem like, in a practical sense, his warning about a large fire nearby would still be worth heeding, even if in a rather dubious highly theoretical sense he wouldn't be talking about a real thing.

Similarly, if your simulated toothbrush still bears the same relations to other things (and e.g. is useful for maintaining your equally simulated dental health), then in a practical sense it does exist. You may have some inaccurate beliefs about it, but do you think that only things that you're in no way mistaken about exist?

Spoiler: additional thought 1

Show

A while ago I considered the difference between the things that Exist and what I called Interactables
I jumped to the conclusion that Interactables were (probably) a subset of Existing things (with the two being identical, if and only if Case 2 was the truth, whole truth and nothing but the truth)
You (Devils Advocate) thought I had written it backward and I was baffled as to why.

Where did I do that?

In step 4, you've created a hypothetical example where you've assumed that the (previously assumed non-existing) object is real

Woah there. I considered an internally consistent hypothetical scenario in which something non-actual would do something. I did not assume that the non-actual thing is actual. It would "exist" in the scenario, but obviously it wouldn't actually exist if actuality is absolute (i.e. if a thing's actuality is independent of universe of discourse, rather than relative to universe of discourse (even though that's of course how universes of discourse work and therefore actuality isn't absolute, and you should acknowledge that)).

Again, if something is logically possible, then at least one internally consistent hypothetical scenario contains it.

When you come back to step 7, you've taken it out of the example.

Well, I could re-work the whole thing to just outright consider the logical possibilities of minds thinking and arguing "I think, therefore I am", rather than the logical possibilities of their existences. Then there would be no need for an additional assumption in step 4. What then, hotshot? :P

[jayem]

Which two questions were those?

Goodness, I thought we'd both given up (and recognised the OP had moved on). Fortunately I hadn't forgotten where we were.

We kind of had three cases, that are kind of mutually exclusive
a) What we see/detect isn't all that 'exists' (at the extreme Multiverse)
b) What we see/detect is all there 'exists' and thate is nothing else 'exists'
c) What we see/detect does not 'exist' (at the extreme Brain in Vat)

The two questions were
Could (a) be the actual situation instead of (b) or (c)?
Could (c) be the actual situation instead of (b) or (a)?
[There is a third question, but we tend to assume that is simple to answer]

Because we were swapping between the two questions without warning, things were getting confused.

As regards the spoilered point 2, it was Veti who said it, sorry.

I think you meant to say that the other way round, but either way I don't buy it. If something interacts, then in principle you can show that it exists; if it doesn't, you can't.

I'll come back to the rest later

[Radar]

Woah there. I considered an internally consistent hypothetical scenario in which something non-actual would do something. I did not assume that the non-actual thing is actual.

While I do not feel fully competent to answer everything and I cannot speak for jayem and what he saw, I think I see the inconsistency exactly here. Namely you assume that a non-actual being (I would rather have a better word, since being implies existence) actualy did something and from that you derive your point. If something s non-actual it cannot take actual actions as far as I can understand.

If I take Harry Potter and write that he said "Cogito ergo sum". Then either he is actual and truly said it, or he is not actual and those words were not said at all by him - I just wrote a hypothetical situation. The connection between actual and non-actual in cause-action scenarios seems to be a bit of a reverse of truth-false relations in logic:

Actual beings can take actual actions.
Actual beings can take non-actual actions. (example: Did you do the dishes? Suuure, I totally did them. Just do not go into the kitchen, ok?)
Non-actual beings cannot take actual actions.
Non-actual beings can take non-actual actions.


The whole problem with "Cogito ergo sum" is that only the person thinking can use it to prove his/her own existence. The proof cannot be shared, since consciousness is an internal experience without any tangible expression on the outside that could not be equally well explained without sentience. Chinese room argument can very well be made against humans and the only reason most people would not take it seriously is that they know of their own sentience (through "Cogito ergo sum") and extrapolate.

[jayem]

While I do not feel fully competent to answer everything and I cannot speak for jayem and what he saw, I think I see the inconsistency exactly here. Namely you assume that a non-actual being (I would rather have a better word, since being implies existence) actualy did something and from that you derive your point. If something s non-actual it cannot take actual actions as far as I can understand.
...
The whole problem with "Cogito ergo sum" is that only the person thinking can use it to prove his/her own existence. The proof cannot be shared, since consciousness is an internal experience without any tangible expression on the outside that could not be equally well explained without sentience. Chinese room argument can very well be made against humans and the only reason most people would not take it seriously is that they know of their own sentience (through "Cogito ergo sum") and extrapolate.

Cheers Radar,
Practically we could of course take the chinese room argument more seriously about ourselves. (Descarte would of course approve, hus conclusion is 'I exist' not 'I exist as I perceive myself'). This would be the case of following the second line of questioning. However while we are doing this we have to be aware that we are arguing equally against both the 'multiverse' and 'wysiwygverse' models and when we reject/pause this line of thought, we can't bring the 'wysiwygverse' back by itself. As a discussion I'm all for it, so long as it doesn't get used for a slight of hand.

I point DA to his point 3 "there nevertheless are internally consistent hypothetical scenarios in which they would exist.", 4. Let us assume, for the sake of argument, that in at least one internally consistent hypothetical scenario, one of those [outside the scenario non-actual] logically possible minds would think and argue "I think, therefore I am".
In this case he has kind of got round the initial argument, by explicitly nesting a scenario (the Cogito, Ergo Sum then has no actual contribution to the argument other than to distract us and him from noticing this) or another premise (that non-actual minds can think).

Were he to (as now) drop that step to effectively go "(the thing that is not actually a thing capable of thinking) thinking", then that has it's own problems. In this case the fake nestling of the scenario is the distraction (the suggestion that it could be a thing that is capable of thinking, only applies for this line).
In which case Top Gun Radar has I think got the better phrased answer.
__


Regarding the simulated toothbrush, no of course I don't think that only things that I am in no way mistaken about exist. After all I came to this thread, arguing that there could be entire existing universes or not, and that neither my nor your opinion on them existing/not had no bearing on whether they exist or not.

I do think it is possible that I am mistaken about the existence of things that I think exist, and in the "brain in the vat" case that would be almost everything (and yes, even were I to know I were a simulation, it would make conversation awkward to actually put appropriate qualifications every time). Conversely (Inversely or Contrapositively) I could be mistaken about the non-existence of things that I think don't exist.

[Devils_Advocate]

Goodness, I thought we'd both given up (and recognised the OP had moved on).

I just took a break from this thread... Okay, I took a long break.

The two questions were
Could (a) be the actual situation instead of (b) or (c)?
Could (c) be the actual situation instead of (b) or (a)?
[There is a third question, but we tend to assume that is simple to answer]

[For sake of completeness, is it the following?]

Could (b) be the actual situation instead of (a) or (c)?

Because we were swapping between the two questions without warning, things were getting confused.

I see the whole discussion more as dealing with one question: What does "exist" even mean? Or, better: What are the various things that "exist" can be used to mean?

While I do not feel fully competent to answer everything and I cannot speak for jayem and what he saw, I think I see the inconsistency exactly here. Namely you assume that a non-actual being (I would rather have a better word, since being implies existence) actualy did something and from that you derive your point.

Not even remotely. If anything, I would assume the described actions to be equally non-actual.

("Entity" might be a superior alternative to "being".)

The whole problem with "Cogito ergo sum" is that only the person thinking can use it to prove his/her own existence.

Whether "I think, therefore I exist" works as a proof of my own existence depends rather heavily on what this existing business is, now doesn't it? Hopefully you don't think that the proof works regardless of what "exist" means.

If various non-actual minds would also think and argue "I think, therefore a I am", then what reason do I have to suppose that I'm an actual mind actually thinking that, rather than that I would think that, but actually don't, because I don't actually exist?

Like, what even is the distinction between the two cases? Riddle me that.

Regarding the simulated toothbrush, no of course I don't think that only things that I am in no way mistaken about exist.

Then why do you think that your toothbrush doesn't exist if it's simulated?

If the world is a computer simulation, then there is still physical matter whose configuration results the perceptions that you associate with the concept of a toothbrush. That configuration of matter may be very different than you thought, but how does that put you in a different position from someone who believes matter to be infinitely divisible, rather than consisting of finite fundamental particles (assuming that the former is incorrect and the latter is correct)?

I do think it is possible that I am mistaken about the existence of things that I think exist, and in the "brain in the vat" case that would be almost everything

Someone might say that by "fire" he meant a process resulting in a release of phlogiston, and thus if no such process exists then what he calls "fire" does not exist. But would that argument by contrived definition seriously mean that his use of the word "fire" failed to refer to something that exists? Again, I raise the practical point that confusion about the exact nature of combustion doesn't necessarily mean that a warning about a fire isn't worth heeding.

[jayem]

I just took a break from this thread... Okay, I took a long break.
[For sake of completeness, is it the following?]
Could (b) be the actual situation instead of (a) or (c)?

I appreciated the break, so thanks. And Yep, spot on.

Obviously that is a lot of a simplification, you could create examples between b&c, "ok I can't trust reports but my immediate perceptions are ok (Truman show world)", and between a&b,
Exactly where 'b' is slightly badly defined. And though I said they were mutually exclusive you could possibly create more complex nested variants. How does Carol from the Truman Show(UK) relate to Bob from Truman Show (US).

I see the whole discussion more as dealing with one question: What does "exist" even mean? Or, better: What are the various things that "exist" can be used to mean?

Again, stating the obvious given my previous posting: I'm happy for there to be multiple definitions (or new phrases). But also feel that none of those 3 cases above can be ruled out scientifically on the basis that we're using a definition of the word linguistically (and variants of "exist" are the best match we have).

I am moderately happy to say my toothbrush exists(2) [in this simulation] but doesn't really exist(3) or that my toothbrush exists(1) but doesn't exist(2) [in this universe]. The minor problem being that exists(2) is the easiest to find weasel words for, but where they are also most inconvenient (because I tend to speak about exist(2)ing things a lot).
[exists(1) relates to case (a) and exists (3) to case (c)]

Also stating the obvious, this thread came across from the question "are multiverses possible in the real world?" To which while the answer of course is a definite no [at least for the sense of multiverse meant] and the relevant question is "Is The real world possibly in a multiverse". We need to be using whatever word and definition is sensible for the question meant, even if we also play with other questions and situations.

Then why do you think that your toothbrush doesn't exist if it's simulated?

Because it's simulated. If it were the only such entity and I were the only one receiving the simulation (Maybe I've been hypnotised), it seems a fairly clear case. You might say it exists only to me, to spare my feelings, if I were in denial of plaque build-up I'd hope you'd hit me on the head and take me to the hospital.

Of course, if we assume brain in vat, the teeth it's simulating cleaning are simulated, this conversation is simulated and the space it's simulated in is simulated. Even from the simulators case, I'd imagine it would be handy to have a word exists(4?) to describe things in my (simulated) reality. And from my perspective, it would be the same as things that exist(2) if (I falsely pretended/believed) my simulation were not a simulation. Again if the simulation were causing ill effects on a unnecessarily starving 'real' me that distinction might be important (if I'm a very complex NPC then...).
Also of course the 'outer world' electronic configuration that corresponds to a toothbrush being in my simulation does exist (I'd imagine a half decent simulation, depending on it's purpose, would compress it, and probably me, pretty tightly, so that messes with things even more)