The prolific Margaret Cavendish (1623–1673) published books on natural philosophy as well as stories, plays, poems, orations, allegories, and letters. Her mature philosophical system offered a unique vitalist materialist theory of Nature as composed of a continuous, non-atomistic, perceiving, knowing matter. In contrast to the dominant philosophical thinking of her day, Cavendish argued that all matter has free will and can choose whether or not to follow Nature’s rules. The Well-Ordered Universe explores the development of Cavendish’s natural philosophy from the atomism of her 1653 Poems, and Fancies to the vitalist materialism of her 1668 Grounds of Natural Philosophy and argues that her natural philosophy, her medical theories, and her social and political philosophy are all informed by an underlying concern with order, regularity, and rule-following. This focus on order reveals interesting connections among apparently disparate elements of Cavendish’s philosophical program, including her views on gender, on animals and the environment, and on sickness and health. Focusing on the role of order in Cavendish’s philosophy also helps reveal some key differences between her natural philosophy and her social and political philosophy, where Cavendish tended to be quite conservative. Cavendish thought that humans’ special desire for public recognition often leads to an unruly ambition, causing humans to disrupt society in ways not seen in the rest of Nature. The Well-Ordered Universe thus defends reading Cavendish as a royalist who endorsed absolute monarchy and a rigid social hierarchy for maintaining order in human society.