This dissertation offers a new interpretation of the metaphysics of two seventeenth-century women philosophers – Margaret Cavendish (1623–1673) and Anne Conway (1631–1679) – and brings to light an unnoticed tradition in seventeenth-century philosophy. I argue that both Cavendish and Conway can be understood as panpsychist-materialist monists: despite their other differences, they agree that there is one kind of substance in nature or creation, and that the single sort of substance always displays material features and mental capacities. Further, I propose that Cavendish and Conway are joined by the physician Francis Glisson (1597–1677) and the poet John Milton (1608–1674) as examples of a distinct panpsychist-materialist tendency in early modern England. ‘Panpsychist-materialist monism’ may at first seem too clunky to serve as the moniker of a movement, but it earns its keep by accurately capturing three elements of the figures’ systems that, when studied together as a group of related commitments, reveal the philosophical significance of each person’s views. My reading therefore bears on the project of interpreting Cavendish and Conway on their own terms and changes the way their context should be understood. Moreover, to the extent that contemporary philosophers of mind draw on philosophers from history in the formulation of their current views, the work presented in this dissertation stands to make a difference in present-day philosophy as well.