Shulamith Firestone, a Canadian-born feminist who wrote the influential book The Dialectic of Sex at age 25, has died. She was 67.

Firestone was found dead on Tuesday in her Manhattan apartment. Her sister, Laya Firestone Seghi, said her death was of natural causes.

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Shulamith Firestone in an image from Elisabeth Subrin's film Shulie. (Jewish Museum)

Firestone had withdrawn from public life in the late 1970s, though she wrote a second book, Airless Spaces, published in 1997 that detailed her struggles with schizophrenia.

Born in Ottawa to an Orthodox Jewish family, "Shulie" Firestone was raised in Kansas City, Mo., and St. Louis.

In the 1960s, after studying fine arts at university in Chicago, she worked with radical feminist groups in New York — including New York Radical Women, the Redstockings and New York Radical Feminists — and wrote some influential feminist essays.

The publication of The Dialectic of Sex: The Case for Feminist Revolution catapulted her onto the forefront of second wave feminism, alongside names such as Kate Millet, Germaine Greer and Betty Friedan.

'To assure the elimination of sexual classes requires the revolt of the underclass (women) and the seizure of control of reproduction: not only the full restoration to women of ownership of their own bodies, but also their (temporary) seizure of control of human fertility.'—Shulamith Firestone

In the book, she applied Marxist theories of class oppression to women, arguing that women will never have equal treatment as long as they must bear children.

"So that just as to assure elimination of economic classes requires the revolt of the underclass (the proletariat) and, in a temporary dictatorship, their seizure of the means of production, so to assure the elimination of sexual classes requires the revolt of the underclass (women) and the seizure of control of reproduction: not only the full restoration to women of ownership of their own bodies, but also their (temporary) seizure of control of human fertility — the new population biology as well as all the social institutions of child-bearing and child-rearing," she wrote.

Firestone argued women should be freed of the expectation that they will bear children, recommending that children be created in laboratories and raised communally. She also advocated widely available contraception and abortion and advocated state support for child-rearing.  

She aimed to break what she called the "tyranny of the biological family."

Her book became part of many college curriculums and was translated into several languages.  

She became reclusive after its publication, and returned to painting, her first love. But she continued to have a following, with the New York Times reporting people offered to pay her rent as she struggled with mental illness.

Seghi called her sister "a brilliant mind and a totally creative person."

Firestone came to renewed attention in 1997 with the release of Shulie, an independent film by Elisabeth Subrin with Firestone portrayed by an actress Kim Soss.

The 37-minute film based on a 1967 documentary that focused on her life as a then-unknown art student,  was shown at the New York Film Festival, the Museum of Modern Art, the Whitney Biennial and elsewhere.  

Firestone is survived by her mother, Kate Firestone Shiftan, two brothers and two sisters.