This paper considers Margaret Cavendish's distinctive anti-mechanist materialism, focusing on her 1664 Philosophical Letters, in which she discusses the views of Hobbes, Descartes, and More, among others. The paper examines Cavendish's views about natural, material souls: the soul of nature, the souls of finite individuals, and the relation between them. After briefly digressing to look at Cavendish's views about divine, supernatural souls, the paper then turns to the reasons for Cavendish's disagreement with mechanist accounts. There are disagreements over the explanation (...) of particular phenomena, but also a broader disagreement over what to take as one's most basic causal model. (shrink)