"Extreme agnosticism" sounds mostly accurate. She will doubt as a matter of principle, but she won't put a probability on that doubt. As for why I believed what I wrote here…

We talked. A lot. It spanned over multiple conversations, for several months, if not over a year. First, I tried to talk about transhumanist things, like mind uploading. She found it impossible sounding, scary, horrible, and sad. We talked about the potential power of science. She seems to think that science isn't omnipotent (sounds true enough), and some specific things, like the understanding of the human soul, seems definitely out of reach. But I don't recall she ever stuck her neck out and flatly said that there's no way science could ever unravel the mysteries of our minds, even in principle (I personally have some doubt, because of the self-referencing involved. But I don't think these difficulties would prevent us from understanding enough low-level mechanisms to effectively emulate a brain.).

We moved on to more basic things, like reductionism. She often "accuses" me of wanting to control everything with math. So I tried to assert that our world is math all the way down, even if it's way too complicated for us to actually use accurate math. But she doesn't seem to make the bridge between the laws of physics and a full human brain. She seems to assert that there is something there that is by nature incomprehensible. But when I call that "magic", she rejects the term.

At some point, I wrote this (French or English depending on your browser settings). I don't think very highly of it, but I thought it would at least serve my point: stopped being called intolerant just because I take the logical step from believing something to asserting that someone who doesn't believe it is mistaken. (Modulo tiny uncertainties.) It didn't work at all. She just found it juvenile, besides the point, and by the way, the colour of my socks and the existence of God are not the same thing, and should not be reasoned about in the same way. My informal formulation of the Auman's agreement theorem also fell on deaf ears.

We had some more fruitless debates, where I believe she doesn't understand me, and where she believe she perfectly understands me, but I cannot perceive her arguments the same way humans can't perceive ultra-violet, which is why I reject them, the same way some ignorant fool would say "there's no such thing as invisible light". This feels very close to saying that I lack some brain circuitry, though I don't think she would actually say that if I asked. But I do feel like I talk to some mystic who claim to have higher perceptions, and I should not call them hallucinations just because I lack them. (Sounds like Freudian psychoanalysis: If you don't believe it, something is wrong with you.) Of course, her high status (being my mother and older than me) doesn't help. Heck, she even said she used to think like me, but got past that. So I'm clearly immature. I suspect she hopes I will understand her when I get older.

So I ended up writing this (French only for now). She hasn't read it so far, but I told her about the first paragraphs (which I roughly translated in my post here). Then she told me there's something wrong with this. (But again, she won't outright contradict me, and say there isn't a world out there.)

By the way, she thinks I believe all that stuff for some "deep reason", which I take to mean "something unrelated to the actual accuracy of such beliefs". She thinks I have some deep fear inside that makes me cling to that. (No kidding: I see small hopes of making a paradise out of our world, and I would give up on them? OK, if it's impossible after all, let us enjoy our short lives. But if there is a possibility, then missing it is unforgivable.) Strangest of all, she sees a contradiction between my humanistic, left-wing, environmentalist ideas, and my consequentialist, positivist, transhumanist ideas.

Now to her credit, I must note that she probably changed her mind about longevity: without taking into account some problems like world population, she wouldn't be against doubling our life expectancy, or more. Living forever seems too much yet, but a couple centuries seems like a good idea. (She is closer to death lately: her aunt, which she loves dearly, starts to have health problems that may prove serious in the coming years, if not months.)