Set in an unknown time in a utopian world named The Blazing World, Margaret Cavendish’s prose fiction published in 1666 introduces a posthuman utopian world. Cavendish philosophizes on a variety of subjects from science and technology to religion and human rights in her narrative. The text is considered the predecessor of science fiction novels as well as utopian and dystopian novels. It is packed with numerous beings that are half human and half animal as well as immaterial spirits and pieces of technology. The nonhuman creations of the writer in the novel have human-like attributes, which challenges the basic assumptions of human that have prevailed since the Enlightenment. These assumptions of human claiming that humans are the only beings with agency, intelligence, and intentionality may be repudiated by Cavendish’s fictional creatures who have these features attributed to men. In addition, they use their agency, intelligence, and intentionality to shape a world that lives in peace and harmony, which makes them even more outstanding entities than humans in the book. This paper aims to analyze Margaret Cavendish’s proto-science fiction text in the light of posthumanism with a post-anthropocentric view to question the ongoing anthropocentric attitude since the Enlightenment.