The British feminist Julie Burchill this week accused transgender activists of being interlopers who "have your cock cut off and then plead special privileges as women," prompting a backlash so fierce that London's Observer newspaper removed the piece from the web.

But while Burchill cast trans women as a threat to feminism, the real threat is attitudes like hers, which could weaken the entire movement.

A subset of radical, essentialist feminists have for decades believed that transgender people are merely deluded or mentally ill for decades. Germaine Greer, whose The Female Eunuch electrified the movement in 1970, complained in 2009 that "other delusions may be challenged, but not a man's delusion that he is female." Sheila Jeffreys, an Australian feminist who has long criticized sex reassignment surgery, told BuzzFeed Shift that "being a woman is not a matter of gender identity. It's not a matter of what's in the head." To her, womanhood comes from being born in a female body "and experiencing the harms of being a woman in a patriarchal society."

Their views would mean less if feminism were simply a niche movement for a certain subset of women with particular views on biology and identity. But it has come to stand for gender justice in general — and, crucially, for bodily self-determination. It can't do that with any legitimacy if it tries to keep trans women out.

"I often feel failed by feminism," says writer and trans advocate Janet Mock. She adds, "Any woman's right to self-identify is a personal freedom I fight for, and those women who claim trans women are not women are perpetuators of gender-based oppression, and all feminists should be upset and moved to action against this."

She believes that all the issues feminists care about, including abortion, matter greatly to trans people too: "Reproductive rights are about body and medical autonomy, our collective and deeply personal right to choose what we want to do to/with our bodies. Trans people and feminists should be building natural alliances here."

This is an issue of consistency as well as inclusion. A central tenet of feminism has long been that a woman and she alone should have control over her body. To argue that the gender of said body isn't covered by this — that it should instead be determined from the outside — is an increasingly untenable position.