When you see an oak tree and when you think about an oak tree it triggers the same series of neural impulses in your brain. Athletes visualize their actions before doing them, and this provides real benefits to achieving those actions. For humans, there is never any "physical oak leaf" there is only ever constructs.

Okay I think I understand what you're trying to say. So let's go back to our hypothetical observer outside the universe, looking in at the source code. (Not that this is possible, but I find it clarifies my thinking and what I'm trying to say.) The human is looking at an oak tree. The observer is looking at the human's brain, and sees that certain neurons are sending signals to other neurons. The human is closing their eyes and visualizing an oak tree. There's a similar but not identical neural pattern going on–I find the subjective experience of visualizing an oak tree using my imagination isn't quite the same as the experience of looking at one, but the neural firing is probably similar.

Now the human keeps their eyes closed, and the outside-the-universe hypothetical observer looks at the oak tree, which is made out of cellulose, not neurons. The oak tree starts to fall down. In the neural representations in the human's head, the oak tree isn't falling down, because last time he looked at it, it was nice and steady. He keeps his eyes closed, and his earplugs in, and the oak tree falls on his head and he dies. Up until the moment he died, there was no falling oak tree in his mental representation. The information had no sensory channel to come in through. Does that mean it didn't exist for him, that there was never any "physical oak tree?" If so, what killed him?

I think the LessWrong overall attitude to this is comparable to a bunch of observers saying "gee, wouldn't it have been nice if he'd kept his eyes open, and noticed the tree was falling, and gotten out of the way?" The philosophy behind it is that you can influence what goes into your mental representations of the world (I'll stop calling it "map" to avoid triggering your 'modern equivalent of the soul' detector). If you keep your eyes closed when walking in the forest (or you don't get around to going to the doctor and getting a mammogram or a colonoscopy, or [insert example here]), you get hit by falling trees (or your cancer doesn't get detected until it's Stage 5, at which point you might as well go straight to palliative care).

For me there's something basically wrong with claiming that something doesn't exist if no human being knows about it. Was the core of the planet solid before any human knew it was molten? Is an asteroid going to decide not to hit the Earth after all, just because no telescopes were pointed outwards to look for it? What we don't know does hurt us. It hurts us plenty.

Granted, the 'map and territory' claim, along with many other analogies rampant on LW, was aimed more at topics where their is fairly clear evidence for a particular position (say, evolution), and people have ideological reasons not to believe it. But it goes just as well for topics where no human being knows anything yet. They're still out there.

In another comment, you said that you don't think hard science is possible. (Forgive me if i'm misquoting.) Since our entire debate has been pretty much philosophy and words, let's go for some specifics. Do you think research in hard science will stop advancing, or that is should stop advancing? If so, why?

When you see an oak tree and when you think about an oak tree it triggers the same series of neural impulses in your brain.

Correct.

For humans, there is never any "physical oak leaf" there is only ever constructs.

Incorrect.

To understand the distinction, note this passage from The Simple Truth:

Frankly, I’m not entirely sure myself where this ‘reality’ business comes from. I can’t create my own reality in the lab, so I must not understand it yet. But occasionally I believe strongly that something is going to happen, and then something else happens instead. I need a name for whatever-it-is that determines my experimental results, so I call it ‘reality’. This ‘reality’ is somehow separate from even my very best hypotheses. Even when I have a simple hypothesis, strongly supported by all the evidence I know, sometimes I’m still surprised. So I need different names for the thingies that determine my predictions and the thingy that determines my experimental results. I call the former thingies ‘belief’, and the latter thingy ‘reality’.

'belief' corresponds to 'map'; 'reality' corresponds to 'territory'.