I need a really solid article proving our sexual dimorphism.

No.

Take it back to reproduction. The root of male and female and the reason the descriptors exist is reproduction.

Undeniable (intersex people are male or female, nor a third sex)

Ask him to what these descriptors apply, if not human reproductive role.

Point out that the descriptors male and female can apply only to gender identity OR the reproductive role, as the two can be in direct opposition.

So for example, he believes gender identity overrides the reproductive role, so the XY person is female, therefore cannot have a male body, the body must be female

(at this point he may say it's a female brain and a male body, in which case you say, what are the objective criteria for a female brain, and what does female mean if it isn't attached to reproductive role, and also note down that he accepts the body is male, for use later on in your thrashing good argument)

So if male and female are completely detached from reproductive role as required by faith in gender identity then there is no 'male' or 'female' meaning for the gender identity to be ascribed to.

At this point he will be forced to describe the female brain in terms gender stereotypes and you win the argument.

I remember a thread where someone was trying to imagine how reproduction in humans might work with a third gamete.

The dating scenario is as amusing to envisage as the mating.

He denies that it's all about biology, because apparently it's more complex than that. Which is where it gets murky, because that way stereotypes lie.

Then he needs to give his definition. I bet he cannot define it.

It is actually quite insulting of him to insinuate that somebody like, say, Peachy Yoghurt, isn't a 'real woman'. It is absurd that some man's projections of what a woman is should be tha arbitrer of saying 'what a woman is'.

Peachy Yoghurt:

Philosophy is all about accuracy!

Is it? Some philosophers manage to cloak this in incomprehensible obscurity.

Hormones, hey?

I used to have somewhat elevated testosterone due to PCOS. (I've no idea what it is now, post menopause).

This merely made me a somewhat spotty female who needed a bit of help to ovulate.

The treatments for hormonal abnormalities are always in the direction of normalising wrt the person's sex. Of which there are two. The treatments are in the direction of aiding fertility, not destroying it.

No woman with PCOS will ever produce a sperm.

If he has a DPhil it should be easy.

Tell him to apply some critical thinking. Tell him to use his damn logic module, replacing 'woman' and 'man' with A and B and do some bloody maths.

Oh, a doctorate in philosophy? Surely he must know that words have definitions? Let him define 'female' and 'male' in the case of mammals. (Spoiler: the type of gamete define it, egg or egg-fertilizing gamete. End of story.).

I disputed that as a conclusion, but lacked the scientific knowledge to say why!

Well that would fall under cell mosaicism (e.g. trisomia 21 can also be a mosaicism, it means not all cells in the body are affected.) Possible mosaicism can happen by absorption of a twin. There is also a recent study out of a woman with a lot of XY cells who had a daughter with again a lot of XY cells in the body.
However, she could only have a daughter, because her body produced an egg=female gamete (she must have had an uterus and a vagina, too, to do that I suppose). Female gamete ->female->woman.

I have a colleague who quite sincerely believes young earth creationism, despite being a physical scientist by training.

I once had a colleague, very Christian, who believed in creationism who worked in directed evolution!!! She basically copied the technique of evolution in the lab that nature had deviced to improve the fitness of proteins (and ultimately organsims) and didn't believe in evolution!!!

A mate said she'd read an article that talked about how some pregnancies might have had a twin that then died (very early, before it becomes a proper foetus) and absorbed into the living foetus, and that the DNA of the dead twin is found in body parts of the living twin when they are adults. The example (I think!) was an adult female having a hysterectomy that was tested and found to have male DNA which was a remnant of the dead embryo.... her argument was "well that shows that we all could have bits of male and bits of female as many pregnancies could have these unknown second embryos" - I disputed that as a conclusion, but lacked the scientific knowledge to say why! it sounds extraordinarily far fetched but came from a reputable publication so am assuming is based on some sort of real scientific theory. Does anyone with actual scientific knowledge have any idea what I'm talking about?!

He talks a lot about hormones.

The definition of male and female still depends on the gamete (egg vs. sperm). The type of gamete is not influenced by hormones (except for a small window during foetal development. But that is all water down the bridge when the Baby pops out).

But, yeah, let's hope he is just winding you up. That would be cruel. But funny.

Oh no, not the fucking hormone washes in utero shite! (His DPhil was deffo not in sciences, was it? Though having said that, I have a colleague who quite sincerely believes young earth creationism, despite being a physical scientist by training. A science background is not actually a magic vaccination against being totally and utterly barking mad.)

Did he ever think that he could get impregnated and bear a child?

If not why not?

This letter, written as a response to an article in the BMJ, is worth looking at, too:

Proposed ‘terminology’ may mislead and fudges the reality of biological sexual dimorphism. Sex is not “assigned”, but determined at conception and in early embryonic life. Biological characteristics of male and female have “historically” been observed at birth and likely this will continue. Sex should not be confused with gender - a social construct. Although internal subjective identity, legal status and external appearance can change or be re-aligned, a person’s underlying biological sex cannot.

www.bmj.com/content/362/bmj.k3371/rr-0

Pedants' Corner here!
Whitehead & Russell, the 3-volume Principa Mathematica, in Volume II:

Pedant alert on the pedantry (pedantry squared?) There then followed several decades of work on how one could prove that second order arithmetic was both sound and complete (roughly speaking, the axioms plus rule of inference only generate true statements, and the axioms plus rule of inference are sufficient to generate all true statements).

Then Gödel came along in 1931 and showed that (I paraphrase wildly at this point) there were statements of arithmetic which could be shown to be true (using metamathematical arguments outwith arithmetic itself) and also shown to be unprovable within arithmetic.

None of which of course has any bearing on the fact that human beings are a sexually dimorphic species. Yes, there are rare people who are intersex - they are (in the statistical sense, not the moral sense) abnormal, and often subfertile, so from an evolutionary point of view "dead ends".

Do you have kids? Ask him how he knew what combination of humans is required to make babies.

And where are his kids on the sex spectrum? And did he expect them to be that? Did it feel like a 50:50 chance, what sex the baby would be or rather a 1:1000000 situation?

Do you have kids? Ask him how he knew what combination of humans is required to make babies.

Just tell him intersex people are male or female. Most people don't know that.

Does he think sex is a spectrum as seems to be trendy at the mo? Even if it was - how would that make twaw true? It would mean someone jumping from one end of the spectrum to the other.

This thread on twitter is good.
twitter.com/jonjonedinburgh/status/1047902447375671296
"There is no spectrum. In animals, gonads are testes or ovaries , gametes are sperm or eggs. There is nothing in between. Some animals have both at the same time, or change over time. Humans are mammals, and no mammals are hermaphrodites." It carries on.

We really need some biologists, or some kind of association on human biologists if there is one, to put out a statement or an article on this. There is so much crap being spoken on this and it is influencing people.

sexandgenderintro.com

I'd direct him to Rebecca Reilly Cooper. Sounds to me like her logical arguments might do more to persuade him than any science
aeon.co/essays/the-idea-that-gender-is-a-spectrum-is-a-new-gender-prison