You made an analogy between AI x-risk and e.g. cold fusion. I pointed out that there's an important disanalogy here: no one is claiming or has claimed that cold

You made an analogy between AI x-risk and e.g. cold fusion. I pointed out that there's an important disanalogy here: no one is claiming or has claimed that cold fusion poses an existential threat. Hence, the nearest cold-fusion claim to any AI x-risk claims is "cold fusion is worth investigating" (which it was, once, and isn't now). It looks to me as if (1) you made an analogy that doesn't really work, then (2) when I pointed out how it doesn't work, (3) you said "look, you're making an analogy that doesn't really work". That doesn't seem very fair. I wouldn't expect hell itself to have physical-world evidence. But the idea of hell doesn't turn up as an isolated thing, it comes as part of a package that also says e.g. that the world is under the constant supervision of an all-powerful, supremely good being, and that I would expect to have physical-world evidence. I have no problem with the principle that extraordinary claims require extraordinary evidence. The difficult thing is deciding which claims count as "extraordinary". A lot of theists would say that atheism is the extraordinary claim, on the grounds that until recently almost everyone believed in a god or gods. (I'm not sure that's actually quite true, but it might be true for e.g. "Western" societies.) I don't agree and I take it you don't either, but once the question's raised you actually have to look at the various claims being made and how plausible they are: you can't just say "look, obviously this claim is extraordinary and that claim isn't". Advocates of AI x-risk might say: it's not an extraordinary claim that AI systems will keep getting more powerful -- they're doing that right now and it's not at all uncommon for technological progress to continue for a while. And it's not an extraordinary claim that they'll get smarter than us along whatever axis you choose to measure -- that's a thing that's happened over and over again in particular domains. And it's not an extraordinary claim that something smarter than us might pose a big threat to our well-being or even our existence; look at what we've done to everything else on the planet. You, on the other hand, would presumably say that actually some or all of those are extraordinary claims. Or perhaps that their conjunction is extraordinary even if the individual conjuncts aren't so bad. Unfortunately, "extraordinary" isn't a term with a precise definition that we know how to check objectively. It's a shorthand for something like "highly improbable given the other things we know" or "highly implausible given the other things we know", and if someone doesn't agree with you that something is an "extraordinary" claim I don't know of any way to convince them that doesn't involve actually engaging with it. (Of course you might not care whether you convince them. If all you want to do is to encourage other people who think AI x-risk is nonsense, saying "extraordinary claim" and "burden of proof" and so on may be plenty sufficient.)