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I don't think universe as a simulation is epistemologically equivalent to solipsism or this "Last Thursdayism" thing (love the name, by the way).

Solipsism is meaningless. What it means to say that there is nothing outside of your mind? Surely you can't predict or control what is going to happen to your experience, so how can you say that everything is "inside" your mind? what that even means? And if you are a solipsist and wanted to predict what is going to happen next, what you should be doing?

You should start to associate causation to patterns in your experience, and observe carefully that patterns. Those patterns behaviour are not decided by you, so, by definition, they are outside your mind.

"Last Thursdayism" have the same problem that all the religions: OK, maybe it's true, but.. why do you choose Thursday, why not Monday? why not one minute ago? It's totally arbitrary.

Simulation argument has serious experimental issues, but I would not discard it yet. Maybe we can think of something.

> Solipsism is meaningless

Not in all contexts. A healthy dose of solipsism is a good thing - in the sense of it giving us some means of defense against what one could call "reality hypnosis", which is giving too much significance to the status quo, i.e. to what is happening (at present). Reality, too, can be "meaningless", and once that has been realized, it often turns out that changing it is not that difficult.

I would argue that solipsism and a universe simulation are actually very close. You observe x and it might be A or B but you have no ground truth on what A or B looks like or how to distinguish them.

In case of solipsism you experience a universe around you but there seems to be nothing you could use to decide whether you are experiencing an independent outside universe through your senses or whether your mind is just hallucinating this whole outside universe.

In case of the universe simulation you experience a universe around you but you have no a priori knowledge what a real universe and what a simulation of a universe is supposed to look like, you seem to be stuck in more or less the same place as with solipsism.

You should start to associate causation to patterns in your experience, and observe carefully that patterns. Those patterns behaviour are not decided by you, so, by definition, they are outside your mind.

I think mind is probably not the right word here because it has the connotation of control, brain might be the better choice because there are things going on you can not control (ignoring whether you actually have any control to begin with). It may have not been clear from my first comment, but I was especially referring to metaphysical solipsism, denying the existence of an independent outer reality.

Simulation argument has serious experimental issues, but I would not discard it yet. Maybe we can think of something.

Personally I just don't think about it for the moment. There is surely the possibility that we make progress one way or another, but every time I try thinking about it, it leads nowhere. Maybe one day we discover the fundamental laws of physics, logical self-consistent and the only self-consistent laws of physics possible. With mathematical proof. Do we now know that our universe is real, it is after all nothing less than the only way it could ever have been?

Or did the simulator just mess with us again? We could be using inconsistent logic all day long, writing wrong mathematical proofs and be teaching them to the next generation in our universities. But just before someone realizes the error, just after we observed the contradicting consequences of our wrong reasoning, the simulator erases the relevant memory and life goes on as if nothing happened.

Unless somebody wants you to find out, you will never find out. The potential enemy in this scenario is just so powerful, you can not win.

"I would argue that solipsism and a universe simulation are actually very close."

I agree with you that from the point of view of testability both are very similar, but, in my view, from the point of view of logic reasoning are almost the opposite.

Solipsism is just bad reasoning or, at most, a semantic trick: I define my mind as everything there is out there, so, obviously, everything is in my mind. But we have science, precisely, to try to understand what it is out there, and "out there" means outside of our minds.

On the other hand, you arrive to the simulation argument by deduction. That's what, testability apart, make it interesting. The reasoning is: if a simulation is possible, then, many are possible. If there are many simulations in the universe, what is more probable? to be in the universe or in a simulation?

I agree that we don't know what to do with that, but you have to recognize that it's a damn good trick of logic.

I define my mind as everything there is out there, so, obviously, everything is in my mind. But we have science, precisely, to try to understand what it is out there, and "out there" means outside of our minds.

But that is the very point of solipsism - because you are experiencing the outside world only through your senses in your mind, you are unable to decide whether the outside world even exists. The two options are 1) your body with your brain and your mind exists in the usual universe and you can learn about the universe through your senses and 2) the usual universe does not exist, only your brain or your body or something along that line actually exists and the entire universe you think exists outside of your body is just something your mind made up, your mind is simulating the universe for you.

So this is not about redefining mind to mean all the universe, it is about emptying the universe until only your body or your brain or whatever remains, dreaming or hallucinating or simulating the thing we just got rid of all day long so that you can not tell the difference.

Well, obviously, we only have access to our senses.

As I have not control through introspection to my sensorial experience, surely, there is something "out there" generating it. That something is what we (or, as we are talking about solipsism, I) are trying to understand.

What is the point of saying that, what is doing it, i's "my mind"? What have we learned doing that except to mud the issue at hand?

I am actually a bit confused, maybe it is just because English is not my first language and I am misunderstanding some nuances of what you are trying to say. So let my try it from another point of view.

I personally don't dream much or at least usually do not remember much or any of it, but in principle walking along a beach and dreaming about walking along a beach should feel pretty much the same until you realize you only dreamed the latter. So more or less identical experiences to you can have two very different origins, one is rooted in some external reality, the other one is just some kind of hallucination.

You can easily inspect the world of the real beach and at least in principle, maybe by lucid dreaming or something like that, you could also inspect the world of the dreamed up beach. If you never realized you were dreaming, both scenarios might be indistinguishable for you and form the reality you are living in, but nonetheless with very different underlying realities. Does that make sense? There is the reality you are experiencing (beach) and there is the reality that is causing your experience, which may (dreamed up beach) or may not (real beach) be the same thing.

"Experimental" is a key here which distinguishes the simulation hypothesis. There are two things we can do:

1. attempt to create a simulated universe 2. make predictions about our universe based on the hypotheis

I would not rule either of these out as futile.

To some extend we are already simulating the universe, from the smallest scales like lattice QCD simulations because we can not analytically solve the problems to the largest scales to test or hypothesizes about the history of the universe and everything in between. But how would this help to shed any light on the problem of a [non-]simulated universe?

And as mentioned before, making predictions seems even more problematic to me. Where would you even start predicting things? Nobody has ever seen a universe and a universe simulator next to each other and could therefore say we should watch out for this or that to tell the two apart. You probably don't even have good reasons to assume that a simulation would be anything like the outside universe, our universe could be the equivalent of a Tetris level running on a phone somewhere in the real universe.

The difficulty I have with the "universe as a simulation" hypothesis is that it pre-supposes a fundamental difference between simulation and reality, which I strongly suspect is not the case. Everything we have observed about our physical reality indicates that it is 1) composed of mathematical structures and 2) it is computable.

Given these characteristics it is clear that the universe we experience could be executed by any system capable of computation [Universal Raspberry Pi if you will :)]. As such, it is highly likely (almost a certainty) that whatever more foundational reality exists outside of our experience it is very different than ours and is responsible for the computational effort that generates the universe we experience. Given that any physical system is capable of computation when properly organized one could easily imagine a higher dimensional structure that did not require an intelligent being to build it that just "happens" to compute this reality.

Would that be considered a "simulation"? Or is a simulation something that requires an intelligent being to build it? Obviously this turns into a bit of semantics.

The idea that the universe is a simulation is just a different way to express the religious impulse.

Actually its just the opposite - most folks who invoke 'the universe is a simulation' view this as a 'scientific' way of positing a creator of the universe that is not God. Rather it is a program being run on some fantastical computer.

Yes, but how does this bring us any closer to answering any of the unanswered questions in Cosmology? I assert that it doesn't.