According to its website, International Women’s Day (marked this past Tuesday) is a celebration of the global “social, economic, cultural and political achievements of women.” The theme for this year’s platitudinal musings — #BreaktheBias — was supposed to be imagining a “diverse, equitable and inclusive” world that’s free from “bias, stereotypes and discrimination.” I have a suggestion for next year’s theme: #WhatIsAWoman?

“What is a woman?” is the question that Anneliese Dodds, the U.K.’s Shadow Minister for Women and Equalities, was unable to answer on BBC Woman’s Hour on International Women’s Day. The presenter, Emma Barnett, gave Dodds multiple chances to answer on behalf of the Labour Party. But after rambling about people who have gone through gender transition “who want to be defined as a woman,” the best Dodds could come up with was, “It does depend what the context is surely.”

Dodds’s confusion did not deter her from writing on Twitter that “Labour will lift women up, not hold them back.” In response, Harry Potter author J. K. Rowling wrote: “This morning you told the British public you literally can’t define what a woman is. What’s the plan, lift up random objects until you find one that rattles?”

Dodds is not the only progressive who struggles to answer this question. For instance, earlier this year, the Daily Wire’s Matt Walsh appeared on an episode of Dr. Phil alongside two transgender activists. “Can you tell me what a woman is?” Walsh asked Ethan, who identified as non-binary transmasculine (though appeared to be a hormonally masculinized woman). “No, I can’t,” replied Ethan. “Because it’s not for me to say. Womanhood looks different for everybody.” Another activist, Addison, who identified as non-binary (though appeared to be a cosmetically feminized man), had a go: “Womanhood is something that is an umbrella term—” at which point Walsh interjected, “That describes what?” “People who identify as a woman,” Addison said. “Identify as what?” Walsh asked. “As a woman,” Addison said. “What is that?” Walsh replied. And round and round they went.

Contrary to the activist claim, what a woman is does not depend on a subjective sense of identity. Quite simply, a woman is an adult human female. A woman belongs to the female sex, which means she has female chromosomes, reproductive organs, and gametes. Sex is observable at birth (and even earlier with ultrasound technology) and detectable long after death by DNA testing. A man may identify as a woman, put on a dress, give himself a female name and pronouns, take estrogen, or even have his penis removed and a pseudo-vagina constructed — but his sex remains unchanged.

Of course, the socialization of the sexes is something different. There have always been feminine men and masculine women — people who do not fit neatly into traditional sex roles. But a feminine man is not any less of a man than a masculine man. And a masculine woman is not any less of a woman than a feminine woman. Suggestions to the contrary reinforce the same gender stereotypes that the International Women’s Day website claims to be against.

Unfortunately, this incoherence carries real-life consequences. In legislation, policies, and rulebooks throughout the Western world, the objective definition of sex is being replaced with the elusive fiction of gender identity. In the U.K., J. K. Rowling has been one of the few public figures to openly criticize the Scottish parliament for introducing a bill that would allow any man who declares himself to be a woman to have access to women-only spaces and services. (I tried asking Scotland’s first minister what a woman is in 2019, but she wouldn’t — or couldn’t — answer.) The justification? “Trans women are women.”

Fairness, as well as safety, is at stake. In sports, women and girls are being displaced by male athletes whose sex-based advantages do not magically disappear the moment they announce their new gender identity. Yet Lia (formerly Will) Thomas, the male UPenn swimmer who is being permitted to dominate the women’s competition, told Sports Illustrated, “I’m a woman, so I belong on the women’s team.” A less credulous reporter might have asked him the obvious follow-up question, “And what is a woman?”

Those genuinely interested in celebrating women’s achievements must be able to define, at bare minimum, the group they are celebrating. Anyone who cannot give a straight answer to the question “What is a woman?” is worse than useless to the cause of women’s rights.