My chapter examines Isaac Watts’s and Catharine Trotter Cockburn’s views concerning the metaphysics of the mind and their underlying accounts of powers and substances. In Philosophical Essays on Various Subjects Watts criticizes Locke’s account of substances and argues for his own preferred account of substance. Watts argues that there is no need to postulate an unknown substratum, as Locke does. Instead, Watts searches for a better explanation of what substances are. His proposal is that bodily substance just is solid extension (...) and that mental substance is identical with the power of thinking. This means that Watts believes that some powers can be substances. I will show how Watts defends his account of substances against various objections. Cockburn was not satisfied by Watts’s account of substance and disagrees with Watts’s understanding of powers. She believes that Watts is too quick to draw metaphysical conclusions. Cockburn takes seriously the limitations of human understanding and emphasizes that humans are ignorant about many metaphysical truths. I end by assessing the strengths and weaknesses of Watts’s and Cockburn’s accounts of powers and substances. (shrink)