NEW YORK — Writer Shulamith Firestone, who published her influential ‘‘The Dialectic of Sex’’ at age 25 and then entered a period of isolation and mental illness, has died at age 67.

Ms. Firestone was found dead Tuesday in her Manhattan apartment, said her sister Laya Firestone Seghi. The death was from natural causes.

‘‘The Dialectic of Sex: The Case for Feminist Revolution’’ was published in 1970 during the height of the women’s liberation movement.

Ms. Firestone applied Marxist analysis to the status of women and argued that true liberation would come only when women were freed from childbearing. In her utopian future, babies would be gestated outside the womb and raised by both sexes. ‘‘The tyranny of the biological family would be broken,’’ she wrote.

The book joined works like Kate Millett’s ‘‘Sexual Politics’’ and Germaine Greer’s ‘‘The Female Eunuch’’ as a 1970s feminist standard and is still assigned in college courses.

She left public life after the book was published and was hospitalized with schizophrenia in the 1980s.

According to her landlord, Bob Perl, she had been dead for about a week when her body was discovered. Ms. Firestone was beloved, Perl said. People often called his office to volunteer to pay the rent. ‘‘Family, friends, and strangers supported her because she so moved them with her work,’’ Perl said.

Ms. Firestone was born to an Orthodox Jewish family in Ottawa and raised in St. Louis. She earned a degree in painting at the School of the Art Institute of Chicago.

She moved to New York in the 1960s and cofounded three feminist groups intended as radical alternatives to mainstream groups such as the National Organization for Women.

Ms. Firestone’s other book was ‘‘Airless Spaces,’’ a fictionalized account of life in and out of psychiatric hospitals, published in 1998.