"it" isn't a single theory.

The argument that Everettian MW is favoured by Solomonoff induction, is flawed.

If the program running the SWE outputs information about all worlds on a single output tape, they are going to have to be concatenated or interleaved somehow. Which means that to make use of the information, you gave to identify the subset of bits relating to your world. That's extra complexity which isn't accounted for because it's being done by hand, as it were..

By far the best definition I’ve ever heard of the supernatural is Richard Carrier’s: A “supernatural” explanation appeals to ontologically basic mental things, mental entities that cannot be reduced to nonmental entities.

Physicalism, materialism, empiricism, and reductionism are clearly similar ideas, but not identical. Carrier's criterion captures something about a supernatural ontology, but nothing about supernatural epistemology. Surely the central claim of natural epistemology is that you have to look...you can't rely on faith , or clear ideas implanted in our minds by God.

it seems that we have very good grounds for excluding supernatural explanations a priori

But making reductionism aprioristic arguably makes it less scientific...at least, what you gain in scientific ontology, you lose in scientific epistemology.

I mean, what would the universe look like if reductionism were false

We wouldn't have reductive explanations of some apparently high level phenomena ... Which we don't.

I previously defined the reductionist thesis as follows: human minds create multi-level models of reality in which high-level patterns and low-level patterns are separately and explicitly represented. A physicist knows Newton’s equation for gravity, Einstein’s equation for gravity, and the derivation of the former as a low-speed approximation of the latter. But these three separate mental representations, are only a convenience of human cognition. It is not that reality itself has an Einstein equation that governs at high speeds, a Newton equation that governs at low speeds, and a “bridging law” that smooths the interface. Reality itself has only a single level, Einsteinian gravity. It is only the Mind Projection Fallacy that makes some people talk as if the higher levels could have a separate existence—different levels of organization can have separate representations in human maps, but the territory itself is a single unified low-level mathematical object. Suppose this were wrong.

Suppose that the Mind Projection Fallacy was not a fallacy, but simply true.

Note that there are four possibilities here...

1. I assume a one level universe, all further details are correct.

2. I assume a one level universe, some details may be incorrect

3. I assume a multi level universe, all further details are correct.

4. I assume a multi level universe, some details may be incorrect.

How do we know that the MPF is actually fallacious, and what does it mean anyway?

If all forms of mind projection projection are wrong, then reductive physicalism is wrong, because quarks, or whatever is ultimately real, should not be mind projected, either.

If no higher level concept should be mind projected, then reducible higher level concepts shouldn't be ...which is not EY's intention.

Well, maybe irreducible high level concepts are the ones that shouldn't be mind projected.

That certainly amounts to disbelieving in non reductionism...but it doesn't have much to do with mind projection. If some examples of mind projection are acceptable , and the unacceptable ones coincide with the ones forbidden by reductivism, then MPF is being used as a Trojan horse for reductionism.

And if reductionism is an obvious truth , it could have stood on its own as apriori truth.

Suppose that a 747 had a fundamental physical existence apart from the quarks making up the 747. What experimental observations would you expect to make, if you found yourself in such a universe?

Science isn't 100% observation,it's a mixture of observation and explanation.

A reductionist ontology is a one level universe: the evidence for it is the success of reductive explanation , the ability to explain higher level phenomena entirely in terms of lower level behaviour. And the existence of explanations is aposteriori, without being observational data, in the usual sense. Explanations are abductive,not inductive or deductive.

As before, you should expect to be able to make reductive explanations of all high level phenomena in a one level universe....if you are sufficiently intelligent. It's like the Laplace's Demon illustration of determinism,only "vertical". If you find yourself unable to make reductive explanations of all phenomena, that might be because you lack the intelligence , or because you are in a non reductive multi level universe or because you haven't had enough time...

Either way, it's doubtful and aposteriori, not certain and apriori.

If you can’t come up with a good answer to that, it’s not observation that’s ruling out “non-reductionist” beliefs, but a priori logical incoherence"

I think I have answered that. I don't need observations to rule it out. Observations-rule it-in, and incoherence-rules-it-out aren't the only options.

People who live in reductionist universes cannot concretely envision non-reductionist universes.

Which is a funny thing to say, since science was non-reductionist till about 100 years ago.

One of the clinching arguments for reductionism.was the Schrödinger equation, which showed that in principle, the whole of chemistry is reducible to physics, while the rise of milecular biology showeds th rreducxibility of Before that, educators would point to the de facto hierarchy of the sciences -- physics, chemistry, biology, psychology, sociology -- as evidence of a multi-layer reality.

Unless the point is about "concretely". What does it mean to concretely envision a reductionist universe? Pehaps it means you imagine all the prima facie layers, and also reductive explanations linking them. But then the non-reductionist universe would require less envisioning, because byit's the same thing without the bridging explanations! Or maybe it means just envisioing huge arrays of quarks. Which you can't do. The reductionist world view , in combination with the limitations of the brain, implies that you pretty much have to use higher level, summarised concepts...and that they are not necessarily wrong.

But now we get to the dilemma: if the staid conventional normal boring understanding of physics and the brain is correct, there’s no way in principle that a human being can concretely envision, and derive testable experimental predictions about, an alternate universe in which things are irreducibly mental. Because, if the boring old normal model is correct, your brain is made of quarks, and so your brain will only be able to envision and concretely predict things that can predicted by quarks.

1. "Your brain is made of quarks" is aposteriori, not apriori.

2. Your brain being made of quarks doesn't imply anything about computability. In fact, the computatbolity of the ultimately correct version of quantum physics is an open question.

3. Incomputability isn't the only thing that implies irreducibility, as @ChronoDas points out.

4. Non reductionism is conceivable, or there would be no need to argue for reductionism.

It gives you the correct probabilities for your future observations, as long as you normalize whatever you have observed to one. The difference from Copenhagen is that in Copenhagen there is a singular past which actually is measure 1.0.

Now what's difficult is figuring out the role of measure in branches which have fully decohered, so that they can no longer observe each other. Wether an "Everett branch" is such a branch is unknown .

Once again, no ontology is actually implied. It’s absolutely trivial to describe behavior of indetermenistic processes in terms of probability experiment. I’m concentrating on deterministic cases simply because they are trickier

If that's what you actually think, the first line should read something like "under circumstances where probability is in the mind".

You keep missing the point

The point is that a map has to represent the territory.

"And sure, every map is, in a sense, a map of the world", as you out it.,

So if a the territory is branching , the map.should, too. (A map may include aspects of human knowledge as well).

I’m proposing a better map, capable to talk about knowledge states and uncertainty, in any circumstances

That's a disadvantage, because the same map can't represent any territory.

Threre may be an ontologically neutral way of doing probability calculations, but it's not a map, for that reason....more of a tool.

If you think that the framework of probability experiment that I’m outlining in the post fails to account for something that the frameworks of possible worlds manage to account for

The problem is the implied ontology. You haven't actually proven that probability is only in the mind and you can't prove it using methodology, because its a statement about the territory , not just about probability calculations.

Possible world is a term from a map. There may be a referent for it in a territory

If there is a referent for it in the territory, it is entirely reasonable to say "possible worlds exist".

But it doesn’t mean that we have to use this particular term to talk about this referent. We may have a better term, instead.

Is it really a win to admit the substance of existing possible worlds, but under a different name?

even if we grant that the universe is utterly deterministic and therefore probability is fully in the map, this map *still *has to correspond to the territory for which you have to go an look

The map that corresponds to a deterministically branching multiversal has possible worlds. The map that corresponds to a Copenhagen universe has inherent indeterminism

What I’m saying is that even the talk itself about “possible worlds”—without assumption of their realism—is harmful as this framework leaves us unable to reason about logical uncertainty

Refusing to.ever talk about possible worlds is dangerous, because they might exist (they do in MWI ) and they might be useful otherwise. What you really have is an argument that they are a poor match for logical uncertainty, which they are, but you are allowed to use different tools for different jobs.

Having dogmatic , non-updatable assumptions is bad (see rationality, passsim) and it's still bad when they are in the direction of determinism, reductionism, etc.

The Born probabilities are in the mind under the MWI! Reality just has the amplitudes

Which are pretty similar. They are objective feature of the territory that tell you how likely you are to see things.

And note that MWI features really existing possible worlds, in that it features a a multiplicity of existing actual worlds...and what is actual is possible. What MWI removes is chance.

And note that the argument from MWI doesn't support "probability is in the Mind" as it is usually stated, because it is usually stated as something that is true unconditionally, and MWI is only one possibility.

A core tenet of Bayesianism is that probability is in the mind

That argument never had anything to do with Bayesianism as known to the Rev. Bayes...it's much more to do with Jaynes and Yudkowsky.

Also, it was never valid...it was pointed out a long time ago that (a form of) probability being in the mind doesn't imply (a form of) it isn't in the territory as well.

Armchair arguments can't prove anything about the territory...you have to look.

The people whose job it is to investigate this sort of thing, physicists , have been unable to decide the issue.

The specific reason for believing in in-the territory randomness is :

Bell's theorem - Wikipedia https://en.m.wikipedia.org/wiki/Bell's_theorem

If..it was pointed out a long time ago that (a form of) probability being in the mind doesn't imply (a firm of) it isn't in the territory as well.

Armchair arguments can't prove anything about the territory...you have to look.

The people whose job it is to investigate this sort of thing, physicists , have been unable to decide the issue.

By the same logic tossing a coin is also deterministic, because if we toss the same coin exactly the same way in exactly the same conditions, the outcome is always the same.

That not true because fundamental determinism is true , or because effective determinism at the macroscopic level is true.

But beyond that, it allows us to get rid of these “possible worlds” which were leading everyone astray. Now instead of speculating about some weird metaphysics that we have no idea about, we explicitly approximate some process in the real world

You may be beating a dead horse there. Talk of possible worlds doesn't have to imply realism about possible worlds, just as mathematical anti-realists can talk about numbers without committing to their mind independent existence.

"In philosophy, possible worlds are usually regarded as real but abstract possibilities (i.e., Platonism),[4] or sometimes as a mere metaphor, abbreviation, or as mathematical devices, or a mere combination of propositions" -- WP.

Tl;Dr: Talk of probabilities and possible worlds doesn't have to be talk about the territory. But it can be.

1. In an infinite universe, there are infinitely many copies of you (infinitely many of which are Boltzmann brains

That might be true if "you" are a snapshot , or observer moment. Long lasting Boltzman brains are vanishingly unlikely, OTOH. Time in general is a problem for multiversal theories.

the least complex description of your conscious experience is the description of an external lawful universe and directions for finding the substructure embodying your experience within that substructure.

Why isn't it solipsism? Why is a large universe plus a long "address" simpler than a small universe plus a short address?

A quantum mechanical state can be described as a linear combination of “classical” configurations

It doesn't have to be, though.

The fact that we are described by algorithm A rather than B is no more or less mysterious than the fact that the laws of physics are like so instead of some other way.

Then you are not actually deriving the Born rule from UDASSA.