Shulamith Firestone, a widely quoted feminist writer who published her arresting first book, “The Dialectic of Sex,” at 25, only to withdraw from public life soon afterward, was found dead on Tuesday in her apartment in the East Village neighborhood of Manhattan. She was 67.

Ms. Firestone apparently died of natural causes, her sister Laya Firestone Seghi said.

Subtitled “The Case for Feminist Revolution,” “The Dialectic of Sex” was published by William Morrow & Company in 1970. In it, Ms. Firestone extended Marxist theories of class oppression to offer a radical analysis of the oppression of women, arguing that sexual inequity springs from the onus of childbearing, which devolves on women by pure biological happenstance.