NEW YORK (AP) — Feminist writer Shulamith (shoo-LAH'-mith) Firestone has died in New York City at age 67.

Firestone was found dead Tuesday in her Manhattan apartment. Her sister Laya Firestone Seghi said Friday that the death was from natural causes.

Firestone was only 25 when she published a radical critique of women's oppression titled "The Dialectic of Sex." She argued that women would be liberated only when they were freed from the burden of childbearing.

The book was a feminist standard in the 1970s and is still assigned in college courses.

But Firestone retreated from public life after the book was published. She was hospitalized with schizophrenia in the 1980s.

Seghi called her sister "a brilliant mind and a totally creative person." She said the death was "a great loss."