I'm mostly writing this stuff trying to explain what my mindset, which I guess to be somewhat coincident with the general LW one, is like, and where it seems to run into problems with trying to understand these theories. My question about the assumptions is basically poking at something like "what's the informal explanation of why this is a good way to approach figuring out reality", which isn't really an easy thing to answer. I'm mostly writing about my own viewpoint instead of addressing the metaphysical theory, since it's easy to write about stuff I already understand, and a lot harder to to try to understand something coming from a different tradition and make meaningful comments about it. Sorry if this feels like dismissing your stuff.

The reason I went on about the complexity of the DNA and the brain is that this is stuff that wasn't really known before the mid-20th century. Most of modern philosophy was being done when people had some idea that the process of life is essentially mechanical and not magical, but no real idea on just how complex the mechanism is. People could still get away with assuming that intelligent thought is not that formally complex around the time of Russell and Wittgenstein, until it started dawning just what a massive hairball of a mess human intelligence working in the real world is after the 1950s. Still, most philosophy seems to be following the same mode of investigation as Wittgenstein or Kant did, despite the sudden unfortunate appearance of a bookshelf full of volumes written by insane aliens between the realm of human thought and basic logic discovered by molecular biologists and cognitive scientists.

I'm not expecting people to rewrite the 100 000 pages of complexity into human mathematics, but I'm always aware that it needs to be dealt with somehow. For one thing, it's a reason to pay more attention to empiricism than philosophy has traditionally done. As in, actually do empirical stuff, not just go "ah, yes, empiricism is indeed a thing, it goes in that slot in the theory". You can't understand raw DNA much, but you can poke people with sticks, see what they do, and get some clues on what's going on with them.

For another thing, being aware of the evolutionary history of humans and the current physical constraints of human cognition and DNA can guide making an actual theory of mind from the ground up. The kludged up and sorta-working naturally evolved version might be equal to 100 000 pages of math, which is quite a lot, but also tells us that we should be able to get where we want without having to write 1 000 000 000 pages of math. A straight-up mysterian could just go, yeah, the human intelligence might be infinitely complex and you'll never come up with the formal theory. Before we knew about DNA, we would have had a harder time coming up with a counterargument.

I keep going on about the basic science stuff, since I have the feeling that the LW style of approaching things basically starts from mid-20th century computer science and natural science, not from the philosophical tradition going back to antiquity, and there's some sort of slight mutual incomprehension between it and modern traditional philosophy. It's a bit like C.P. Snow's Two Cultures thing. Many philosophers seem to be from Culture One, while LW is people from Culture Two trying to set up a philosophy of their own. Some key posts about LW's problems with philosophy are probably Against Modal Logics and A Diseased Discipline. Also there's the book Good and Real, which is philosophy being done by a computer scientist and which LW folk seem to find approachable.

The key ideas in the LW approach are that you're running on top of a massive hairball of junky evolved cognitive machinery that will trip you up at any chance you get, so you'll need to practice empirical science to figure out what's actually going on with life, plain old thinking hard won't help since that'll just lead to your broken head machinery tripping you up again, and that the end result of what you're trying to do should be a computable algorithm. Neither of these things show up in traditional philosophy, since traditional philosophy got started before there was computer science or cognitive science or molecular biology. So LessWrongers will be confused about non-empirical attempts to get to the bottom of real-world stuff and they will be confused if the get to the bottom attempt doesn't look like it will end up being an algorithm.

I'm not saying this approach is better. Philosophers obviously spend a long time working through their stuff, and what I am doing here is basically just picking low-hanging fruits from science that's so recent that it hasn't percolated into the cultural background thought yet. But we are living in interesting times when philosophers can stay mulling through the conceptual analysis, and then all of a sudden scientists will barge in and go, hey, we were doing some empiric stuff with machines, and it turns out conterfactual worlds are actually sort of real.