Cavendish Margaret, Duchess of Newcastle (1623–1673), English author of some dozen works in a variety of forms. Her central philosophical interest was the developments in natural science of her day. Her earliest works endorsed a kind of atomism, but her settled view, in Philosophical Letters (1664), Observations upon Experimental Philosophy (1666), and Grounds of Natural Philosophy (1668), was a kind of organic materialism. Cavendish argues for a hierarchy of increasingly fine matter, capable of self-motion. Philosophical Letters, among other matters, raises problems for the notion of inert matter found in Descartes, and Observations upon Experimental Philosophy criticizes microscopists such as Hooke for committing a double error, first of preferring the distortions introduced by instruments to unaided vision and second of preferring sense to reason. See also ORGANIS. M.At.