Thanks. This exchange really awakened me, not so much to the fact that MWI is 'wrong' (I still don't think it is), but to the enormity of the philosophical challenge one faces in justifying it.
To clarify, there are two senses in which MWI could be 'right' or 'wrong'.
(a) Do we in fact live in a multiverse?
(b) Could we have found ourselves in a multiverse? That is, in a hypothetical universe where objective physical reality is exactly as MWI theorists describe it, and supposing it contained structures that behaved like conscious observers, could we successfully predict 'what it would be like' to be those observers, and would it be 'essentially' like our own experiences (whatever we deem the 'essential' aspects of our experience to be)? (From my perspective, this is the same as asking: If you did a simulation of an MWI universe on an ordinary computer, would the 'simulated beings' have experiences like ours (in the relevant respects)?)
Leaving aside (a) for the moment, I think the 'disconnect' between our respective worldviews works out like this:
I think that, since nonrelativistic QM is a self-contained theory, if we're given a model of this theory - a 'toy universe' if you like - it must be possible to deduce a priori what conscious observers there are in this universe, if any, and what they are conscious of (albeit with my usual caveat that we shouldn't expect perfectly "sharp" answers.) If you accept this premise, then you don't need to poison the ontological simplicity of MWI by adding 'Worlds', 'Histories' or 'Minds'. All you need to do is carefully 'unpack' what's already in front of you. (The "Bare Theory" is enough, modulo a lot of analytic reasoning.)
(This presupposes the truth of analytic functionalism.)
On the other hand, I suspect you regard it as obvious - so obvious that it should go without saying - that the Bare Theory isn't enough. That the work you need to do in order to generate empirical predictions must be 'synthetic' rather than 'analytic'. That bringing 'many worlds' to life requires axioms rather than mere definitions.
Your experience definitely exists - it is, after all, the only reason you know that anything exists; it is, by hypothesis, not all that exists; and so, within That Which Exists, there is a sharp and objective distinction between That Which You Experience and That Which You Do Not Experience.
Sorry but I'm unpersuaded. I mean, when a hill gets smaller it gradually and continuously loses its identity as a 'thing', to the local topographical variations around it. Likewise, the 'boundary' of a hill is somewhat indeterminate (just as the boundary of the Sun's gravity well is somewhat indeterminate).
It should also be apparent that you don't just exist or experience featurelessly; you are something, a particular entity, and you are experiencing something particular. This is what you are denying when you say that there are situations where there is no fact about what your state of consciousness is or even whether you have one. To which all I will say is that that is the ultimate in irrationalism.
The ultimate in irrationalism? To me those conclusions look unavoidable. Don't Dennett's examples make any impression on you? Consider change blindness: you're alternately shown two slides A and B which depict the same scene, but with a significant alteration which under normal circumstances would 'leap out at you'. But because there's a short delay between presentations of A and B, the slides can be switched many times before you notice the difference. Then Dennett wants to say that there's no 'right answer' to the question of whether, prior to noticing the change, your visual phenomena included the bit of the scene that was changing.
And doesn't pointing out the inevitable arbitariness of any attempt to single out 'the first appearance of consciousness in the tree of life' cut any ice? To me it's the most obvious thing in the world that there isn't always a determinate answer to the question "is that thing conscious?" I honestly find it baffling that so many thinkers resist this conclusion.
For a rather technical example of this (but worth a look if you want to see the real thing), see the latest paper by Richard Borcherds.
Thanks for the reference - I'm always meaning to teach myself about this stuff.
User DZS has directed me to a paper which provides a relativistically covariant ontology for QFT! Consistent histories already had this, but the histories there can be arbitrarily sparsely specified. This ontology, "space-time state realism", goes to the opposite extreme and it does so very ingeniously. It uses the "Heisenberg picture" of QFT rather than the "Schrodinger picture". The Schrodinger picture is the usual one in which the state vector evolves in time and the operators do not. The Heisenberg picture is usually descr...
Permutation City is an awesome novel that was written in 1994. Even if the author, Greg Egan, used a caricature of this community as a bad guy in a more recent novel, his work is still a major influence on a lot of people around these parts who have read it. It dissolves so many questions around uploading and simulation that it's hard for someone who has read the book to talk about simulationist metaphysics without wanting to reference the novel... but doing that runs into constraints imposed by spoiler etiquette.
So go read Permutation City if you haven't read it already because it's philosophically important and a reasonably fun read.
In the meantime, if you haven't then you should also read A Fire Upon The Deep by Vernor Vinge (of "singularity" coining fame) and then read Eliezer's fan fic The Finale of the Ultimate Meta Mega Crossover which references both of them in interesting ways to make substantive philosophical points and doesn't take too long to read.
In the comments below there will be discussion that has spoilers for all three works.