This post demonstrates a deep misunderstanding of modal logics, and of the notions of possibility and necessity. one would expect that misunderstanding given that Eli can't really get himself to read philosophy. For example:

"I have to make an AI out of electrons, in this one actual world. I can't make the AI out of possibility-stuff, because I can't order a possible transistor."

What? What kind of nonsense is this? No contemporary philosophers would ever say that you can make something out of "possibility stuff", whatever the hell that is is supposed to be.

Or this:

"It's going to be because the non-ontologically-fundamental construct of "possibility" turns out to play a useful role in modeling and manipulating the one real world, a world that does not contain any fundamentally possible things."

Eli, everything that is actual is trivially possible, according to every single contemporary analytic philosopher. I have no idea what you mean by "fundamentally possible", but I doubt you mean anything useful by it. If x exists, then it's possible that x exists. If x is an actual object, then x is a possible object. If you want, you can treat those claims as axioms. What's your beef with them? Surely you don't think, absurdly, that if x actually exists then it's not possible that x exists?

One also has to wonder what your beef with meaning is. I mean, surely you mean something and mean to communicate something when you string lots of letters together. Is there nothing you mean by "reductionism"? If you don't mean anything by using that linguistic term, then nobody should pay attention to you.,

I'm hardly claiming that if we find some true future unified theory of physics, every physical proposition we current believe is false. To assert that would be absurd. The interesting question is which of the propositions that are part of the current packages (at least one of which is false) are in fact false.

If you want to pick some of those propositions and rely upon them, you'd better have independent evidence for their truth (the accurate predictions made by the package isn't going to count). So rely on that as your evidence, and not on the false package. This is what less wrong people would do.

Barbour is engage more in philosophy than in hard science, and his work is published in either published in poor journals or book form. I could care less whether Eliezer endorse Barbour's views, if he does. And if he does, then maybe he'd do better to rely upon them than upon QM. If Eliezer does advocate for Barbour's views, one must wonder why. On the basis of some nice hard scientific evidence? Or on the basis of lots of wishful thinking?

I'm not very interested in getting into a debate about how to properly taxonomize physical theories. That's irrelevant to any of the points I've made, and the debate would be even more irrelevant. Two paragraphs of your response are on this irrelevant subject matter.

It is perfectly acceptable for me to bundle the whole of QM into one proposition, if Eliezer is baselessly relying upon QM. If Eliezer wants to rely on something OTHER than QM, then he can. Something OTHER than QM would be a part of the theory (but not the whole of the theory), or something else like a theory of quantum gravity. If he wants to rely on those things, fine. Then we'll see what the evidence is for what he relies upon. What, for example, is the evidence for treating Barbour's odd-ball theories to be more likely true than, say, a fixed-foliation quantum gravity? Well, there isn't really much evidence. Bad Eliezer.

Your last comment is well worth remarking on. By "very probably false", I roughly meant that there is a 50% probability that it is false. If there is a 50% epistemic probability that P is false, and your belief that Q is based solely upon your belief that P, then you are irrational if you believe that Q.

What you put in for P and Q is irrelevant, for a simple reason. If you're appealing to Q as your only evidence for P, and Q is probably false, then you don't have good evidence for for P. If Eliezer wants to appeal to some Q as his only evidence for P, and Q is probably false, then he has failed.

Of course, if you have independent evidence for P, then you don't need to appeal to P as your evidence for Q (and you shouldn't, since P is very probably false). Here you can appeal to the independent evidence. For example, there is evidence that the earth orbits the sun that is independent of Newton's theory of gravity. It's for that reason that you find your toy examples plausible.

This doesn't work when we're talking about QM. QM is a package deal that makes predictions. Evidence for the truth of many parts of the package come from the accurate predictions the package makes.

Where there is independent evidence for the parts of the QM package Eliezer wants to appeal to, he should be appealing to those parts of the package and rely upon the independent evidence for them. Appealing to QM is just not rationally acceptable behavior for any reasonably informed persons.

Eliezer might well benefit by thinking about the above-linked SEP article in which this claim is broached: "We now appear to have an interesting situation. Quantum mechanics is compatible with two distinct metaphysical ‘packages’, one in which the particles are regarded as individuals and one in which they are not. Thus, we have a form of ‘underdetermination’ of the metaphysics by the physics (see van Fraassen 1985 and 1991; French 1989a; Huggett 1997). This has implications for the broader issue of realism within the philosophy of science. If asked to spell out her beliefs, the realist will point to currently accepted fundamental physics, such as quantum mechanics, and insist that the world is, at least approximately, however the physics says it is. Of course, there are the well-known problems of ontological change (giving rise to the so-called pessimistic meta-induction) and underdetermination of theories by the data. However, the above underdetermination of metaphysical packages seems to pose an even more fundamental problem, as the physics involved is well entrenched and the difference in the metaphysics seemingly as wide as it could be. These packages support dramatically different world-views: one in which quantal particles are individuals and one in which they are not. The realist must then face the question: which package corresponds to the world? The physics itself can offer no help whatsoever and any justification for choosing one package over the other which appeals to metaphysical considerations, for example, runs the risk of drastically watering down the science in scientific realism."

I didn't want to come out and talk about haecceitistic properties, since that would have made me sound even weirder (and it is controversial whether there are such), and I was already presenting some arguments in a hostile environment. But I had such properties in mind when responding. Thanks for providing the SEP link.

I can't predict what will have to change to get a scientific theory that is correct. Sorry. I'm also not interested in arguing for a theory of identity here. I'm just pointing out that Eliezer's argument against a particular theory of identity fails at being less wrong. I don't have to defend a theory in order to provide a perfectly coherent rebuttal.

Relying upon Wikipedia is not advised here. QM and GR, if you stick them together, entail everything. (On the assumption that from a contradiction one can derive anything. Paraconsistent logical systems deny this assumption.) For some proposition, sentence, statement or utterance that P, QM entails P. GR entails not-P. Absent abandoning classical logic (and moving to something like paraconsistent logic), GR and QM are inconsistent.

Let's assume that a theory is false if the theory entails P and not-P (that is, let's ignore paraconsistent logical sytstems). Then sticking GR and QM together entails P and not-P. Any theory that entails both P and not-P is false. So sticking them together fails.

Almost all physicists are happy with the above claims, and so there is an ongoing search for theories that preserve what's supposedly right about QM with what's supposedly right about GR. Enter theories of quantum gravity. These theories might be in some respects "quantum mechanical". That is, they preserve some aspects of QM. These theories aren't QM or GR, however. They're attempts to preserve what's right (let's suppose) about QM and make that compatible with what's right (let's suppose) about GR.

We're utterly in the dark about which such theories might be true. Sadly, that's the state of the game. If you appeal to QM in defense of some interesting claim, you are failing to appeal to a theory you ought, as an intelligent and well-educated person, place a high degree of credence in. Here our favorite blogger is is screwing up.

Thanks for the points. Yes, ArXiv frequently sucks. And people who argue that the set of real numbers has the same cardinality as the set of natural numbers are morons.. =)