The answer is of course straightforward: a woman is an adult human female. But it is nonetheless interesting to take a trip for the through chain of events scientific, political, and cultural that led the West down this road of madness.

The modern claim that a person can become a man or woman through self-identification did not come from a single source or event. It emerged from the fusion of several different cultural and political movements.

Its earliest medical roots lie in European sexology. In the late 19th and early 20th centuries, doctors began documenting people who felt themselves to belong to the opposite sex. German physician Magnus Hirschfeld separated this phenomenon from homosexuality and established Berlin's Institute for Sexual Science in 1919. Some of the earliest hormonal and surgical transitions were conducted through his institute.

The decisive conceptual step came after the Second World War, when psychologists such as John Money distinguished biological sex from gender roles and gender identity. Money argued that a person's psychological identity as male or female could be shaped independently of the body, particularly through childhood upbringing. His most famous attempt to demonstrate this, the David Reimer case, ended disastrously and substantially weakened his stronger claims about gender being socially imposed. Nevertheless, the distinction between sex and gender survived and became extremely influential in universities and medicine.

Feminist theory then carried the distinction into politics. Simone de Beauvoir argued that society turns biologically female people into socially defined women. During the 1960s and 1970s, feminists increasingly used sex to describe biology and gender to describe the expectations, roles, and inequalities imposed upon men and women. Their original purpose was generally to challenge stereotypes, rather than to claim that biological sex itself could be changed.

The more radical leap occurred in late 20th century postmodern and queer theory. Thinkers such as Judith Butler questioned whether even the categories of male and female should be treated as fixed natural categories. Gender came to be understood less as a description of the body and more as an identity produced and expressed through language, behaviour, and social recognition.

Transgender activism subsequently converted these academic theories into a political and legal programme. The older proposition that a small number of people experience severe discomfort with their sex gradually became the broader proposition that everyone possesses an internal gender identity, that this identity may override biological sex, and that governments and other people should recognise the declared identity. The word transgender itself only acquired its current umbrella meaning during the late 20th century.

From the 2000s onwards, universities, professional associations, corporations, schools, courts and international institutions increasingly adopted this language. The World Health Organisation later removed gender incongruence from the mental and behavioural disorders section of the International Classification of Diseases, its global diagnostic standard, and placed it in the chapter on sexual health. It nevertheless retained the diagnosis to support access to appropriate medical treatment.

The present culture war arose because several very different propositions became blurred together. One is the observable fact that some people experience persistent distress or identification inconsistent with their sex. Another is the social convention of allowing such people to live as the opposite sex. A third and far more contentious proposition is that subjective identity literally determines whether somebody is a man or woman for every legal, linguistic, sporting, and biological purpose.

That final proposition is primarily a philosophical, linguistic, and political claim rather than a scientific discovery showing that human reproductive sex can be altered. The conflict became especially contentious when a principle originally concerned with compassion and personal freedom expanded into debates over language, women's sport, prisons, changing rooms, childhood medicine, and the legal definition of sex.

That there are documented biological outliers and conflicting political and ideological movements does not change the fact that sex is an objective biological reality determined by reproductive biology, chromosomes, and the body's organisation for reproduction, even though there are rare disorders of sexual development. Gender roles and personality traits may vary but they do not change a person's sex.