The Duchess of Newcastle was a prolific authoress whose writings include several works on philosophical subjects. Her interest in natural philosophy was encouraged by her husband, William Cavendish, Duke of Newcastle (formerly Marquess), and his brother, Sir Charles Cavendish. Through the Cavendish circle she met both Descartes and Hobbes, although she denied having any significant contact with either. She was a beneficiary of the Cartesian turn in philosophy to the extent that she repudiated traditional book learning and took the thinking self as her point of departure. Her own philosophy is, however, fundamentally opposed to Cartesianism, since she denied dualism of mind and body, proposing instead a materialist and vitalist account of nature. In her Philosophical Letters (1664) and her Observations upon Experimental Philosophy (1666), she made specific criticisms of Descartes, particularly of his dualism – for example, she questioned how an immaterial substance could move a solid body, and his locating the human soul in the pineal gland.

See also Cavendish, William; Dualism; Hobbes, Thomas; Human Being; Pineal Gland