"So to paraphrase, the trans person is naturally one sex in the brain and one sex in the body. Neither is more their "real" sex than the other, and if medical science allowed we could "correct" that either way, but given the current state of science, the only way to "correct" the mismatch is to adjust the body to be closer to what the mind expects. Is that right?"
This is getting into philosophical territory now, but I think most people would prefer to regard their psychological self as their "real" self, for various reasons... In general, though, yes, that is correct... :)
"I should probably note here that you seem to be saying, "yes, it is the body that makes you a woman, but some people feel like a woman and so they need to aquire that body to settle a discomfort in their mind". That isn't actually quite the same as the Genderist "a woman is anyone who says they are a woman, the body/genitals aren't part of that" argument that I was asking about originally, and it seems maybe you would also disagree with them?"
The whole concept of "genderists" and everything associated with it is a fiction made up by GC people as a strawman to argue against. You won't find anyone in the trans community who is anything like that in reality.
On the topic of who is a woman specifically I think I can very easily answer that by asking you a question; If you were forceably transitioned to male against your will, would you no longer consider yourself a woman?
"Your explanation does still raise some questions/challenges - the obvious one being that a sex "change" really isn't possible either - it's a cosmetic approximation of the outside of the opposite sex. It's not really the same underneath. So is that body "inventory" only concerned with the outside of the body? And if the driver here is to relieve the discomfort of the "wrong" body, why do so many trans women keep their male genitals? And (I'm sorry to be so indelicate here) if a trans woman's brain doesn't expect her to have a penis, how come she's able to do stuff like control her pee? I know that sounds like a joke but if you think about the brain is obviously aware there's something there and knows how to control it!"
Well this is a lot... So, first of all, a sex change is not cosmetic, it changes everything right down to the DNA, that's how powerful hormones are... The body inventory is concerned with the whole body and hormones change the whole body so that works out fine.
As far as trans women keeping the bits, there's two things going on... Firstly, it's possible to have differing levels of dysphoria about different parts of your body. It's entirely possible for a trans woman to be extremely distressed about not having breasts, but not really bothered about having a penis. Since we are, after all, talking about a major surgery it's not surprising that someone who feels ambivilent about their bits might skip it. Secondly, nonbinary people do exist. That's a very big topic on it's own and I'm not quite as knowlegable about it cos I'm very binary myself, but for now, suffice to say there are some people who's body inventory tells them they ought to have physical features from both genders.
Finally, what you said about penises... Ummm... No offense but I don't think you really know how penises work? They don't actually move or anything, they just sort of hang there? There isn't really anything that you can "do" with them... Peeing is controlled by the bladder and that's the same regardless of gender so it works the same way, it just has a longer "straw" attached for lack of a better word...
"And why does this body inventory only go wrong when it comes to sex? Why don't we hear about people whose brain tells them they should have been a foot taller? (There's anorexia obviously but clearly that is treated by changing the brain so it must be different.)"
The prevailing theory is as follows: An embryo in the womb recieves two doses of what's called "anti-mullerian hormone" that are produced by the SRY gene (the infamous "male gene" that is associated with the Y chromosome.) The anti-mullerian hormone changes the development of the mullerian ducts (the embryonic sex organs) hence the name. If the embryo gets a dose of anti-mullerian hormone the mullerian ducts develop into the relatively simple anatomy of a male. If they don't the considerably more complex womb develops. The interesting part is the second dose. Why two doses when one will change the mullerian ducts just fine? The answer is, the first dose is produced at two months, and at two months the embryo doesn't have a brain yet. It simply hasn't grown. The second dose is produced at four months, when the brain has grown. The theory is that the second dose causes the brain to adjust to match the physical biology. The consequences of this are obvious. Two doses of the special hormone and you're a cis man. None and you're a cis woman. But if you get the first but not the second you're a trans woman, and if you get the second but not the first you're a trans man.
That's the theory, anyway. Obviously it's impossible to prove, cos the only way to prove it would be to experiment on an embryo with the intent of making the resultant child trans, which would obviously be wildly unethical, but given the peculiarities of the anti-mullerian hormone it seems pretty convincing to me... :)
"Obviously someone whose brain/body image has developed in a body with (for example) two arms and then loses one will experience a mismatch between the mind's expected body and the actual body (and of course in this case there are severed nerves with a physical effect). But that's not really equivalent to a trans person who you say was born with the wrong inventory, so the comparator is really people missing a limb from birth. Do they also suffer phantom limbs?
Well I did actually look into it. Apparently it's not usual but does sometimes happen. But what is really interesting is that phantom limb syndrome can actually be induced by exposing people to doctored images/film of themselves with extra limbs! So the body's mental inventory/self image isn't fixed but is actually being adjusted based on what the mind is being exposed to. (Brains IMO are absolutely amazing)."
The phrase used in the diagnostic criteria for gender incongruence (the proper term for what we're talking about) is "persistent, insistent, consistent." If your mind is persistently, consistently, insistently telling you your body is wrong then you're trans. That's what separates being trans from more ephemeral shenanigans like the one you described.
" Sorry, I appreciate that's a whole barrage of questions...like I said I find brains and how the imaginery self is constructed inside them amazing."
Totally fine! You're much more interesting to talk to than the more snobby and disrespectful types! :)