At her best, Sontag refused to truncate herself in the interest of legibility or to simplify her thinking in the service of easy answers. At her worst, she was dodgy and noncommittal. For the duration of her romance with the photographer Annie Leibovitz, which lasted from 1989 until Sontag’s death in 2004, she never publicly identified as a lesbian. An acquaintance, the formidable critic Terry Castle, recalls that Sontag’s “usual line (indignant and aggrieved) was that she didn’t believe in ‘labels.’” It is easy to see why this cliché might have rankled queer activists — and why “labels” might belong in scornful scare quotes. But whether Sontag’s silence was misguided as a matter of political strategy, it was part and parcel of her earnest commitment to androgyny as a higher form of life.

“The aim of struggle should not be to protect the differences between the two sexes but to undermine them,” she writes in “On Women,” an indispensable new volume edited by her son, David Rieff. The seven reflections on feminism that make up the book — some of them essays, some interviews — were published in various venues in the early 1970s, but most have since become difficult to track down. The only version of “The Double Standard of Aging” that I could find when I searched several years ago was a smudgy scan on a website of dubious legality. Sontag is best known as a meticulous critic of high culture, but there is a reason I was so desperate for a copy of her bracing meditation on the “humiliating process of gradual sexual disqualification” that women undergo as they age. Her writing on modernist novels and foreign-language film is erudite, challenging and occasionally ponderous; her writing on women is crisp and cutting. Heretically, I prefer the latter.

In “On Women,” Sontag is concerned with both limiting and expanding the scope of feminism. Limiting, because she opposes the movement’s incursion into every facet of discussion. “Fascinating Fascism,” a probing appraisal of Nazi filmmaker Leni Riefenstahl, would be a bit of an outlier in the book, if not for the exchange with the poet and feminist theorist Adrienne Rich that it so famously prompted. In a letter to the editor of the New York Review of Books, where the piece first appeared in 1975, Rich faulted Sontag for neglecting to emphasize the connection between fascism and patriarchy. “One is not looking for a ‘line’ of propaganda or a ‘correct’ position,” she wrote, as an unconvincing prelude to doing just that. Sontag’s reply is satisfying in the manner of neatly arranged shelves, efficiently packed suitcases and other emblems of exactitude. In response to Rich’s question as to how the “same mind” could have produced both “Fascinating Fascism” and more explicitly feminist writings, she replied: “Easy. By addressing itself to a different problem, with the intention of making a different point.” In a 1975 interview in the literary magazine Salmagundi, also included in “On Women,” she clarified:

I’d like to see a few platoons of intellectuals who are also feminists doing their bit in the war against misogyny in their own way, letting the feminist implications be residual or implicit in their work, without risking being charged by their sisters with desertion. I don’t like party lines. They make for intellectual monotony and bad prose.

But Sontag’s belief that feminism was not the be all and end all of intellectual life did not mean she was a tepid ally. In one of the most exhilarating passages in “On Women,” she roars:

Women should lobby, demonstrate, march. They should take karate lessons. They should whistle at men in the streets, raid beauty parlors, picket toy manufacturers who produce sexist toys, convert in sizable numbers to militant lesbianism, operate their own free psychiatric and abortion clinics, provide feminist divorce counseling, establish makeup withdrawal centers, adopt their mothers’ family names as their last names, deface billboard advertising that insults women, disrupt public events by singing in honor of the docile wives of male celebrities and politicians, collect pledges to renounce alimony and giggling, bring lawsuits for defamation against the mass-circulation ‘women’s magazines,’ conduct telephone harassment campaigns against male psychiatrists who have sexual relations with their women patients, organize beauty contests for men, put up feminist candidates for all public offices.

Feminism may not be the only appropriate topic of conversation, or the answer to every question, but it must nonetheless be imported into even the most intimate sanctums. “Liberal” feminism, which operates at the level of public policy, is powerless against the sexism that is “expressed on all levels of human interchange, not just in laws but in the detail of everyday life.” Sexism of this sort prevails “in forms of politeness and in the conventions (clothes, gestures, etc.) which polarize sexual identity, and in the flow of images (in art, news, and advertising) which perpetuate sexist stereotypes.” It seeps, like a gas, into every crevice of consciousness.

One set of norms that particularly worried Sontag — and to which she devoted three of the essays in “On Women” — concerns beauty, which is “women’s business in this society” as well as “the theater of their enslavement.” The indignity of aging in a culture that fetishizes female youth is not unrelated to class, as Sontag acknowledges. But even if economic justice could equalize access to the creams and poultices purported to reverse the creep of the years, it could not overturn the widespread conviction that “only one standard of female beauty is sanctioned: the girl.”

Another problem that remains impervious to legal and economic solutions involves the fraught influx of women into the workforce. Sontag maintained that the crux of the issue was not only that women did not have the means to support themselves (and thereby escape their abusive husbands), but also that they tended to flock to stereotypically feminine occupations. “The colonialized status of women is confirmed and indeed strengthened by the sexist division of labor,” by women’s ghettoization in jobs that are “public transcriptions of the servicing and nurturing roles that women have in family life.” Work must therefore become “fully desegregated sexually.”

It is a delight to watch such an agile mind slicing through the flab of lazy thinking. To those who claim that women have different natures than men, Sontag replies succinctly: “The argument from ‘nature’ is self-confirming. Individual lives which do not confirm the argument will always be taken as exceptions, thereby leaving the stereotypes intact.” To those who “deny they believe these differences make women inferior,” Sontag responds:

Their argument is as dishonest as the separate-but-equal argument once used to defend the legal segregation of the races in schools. For the specific content of these supposedly innate differences between women and men imply a scale of values in which the qualities assigned to women are clearly less estimable.

The aim of feminism must be to “abolish the mystique of ‘nature,’” she wrote, to “work toward an end to all stereotyping of any kind.” The wholesale abolition of gender is our only hope of escaping its cruelties and curtailments.

Whether Sontag’s defiant uncategorizability strikes you as subtlety or evasiveness depends on your stomach for uncertainty. “No position can be a comfortable one or should be complacently held,” she said in the Salmagundi interview. The most important debates of the era must remain knotty, “unresolved.” For my part, I prefer conclusiveness in argument — but in life, I am greatly moved by the image of a world in which everyone has the chance to embody the “mental hermaphroditism” that allowed Colette and Sontag to be so intricate, so multiple, so ample and so whole.

Becca Rothfeld is the nonfiction book critic for The Washington Post.