This book offers a systematic and sustained attempt to address the interpretive challenge of Locke's philosophy of mind. Locke has been considered a Cartesian dualist, a reductive materialist, and even an idealist. These conflicting interpretations have given rise to the widespread accusation that Locke is inconsistent on the mind-body issue. This book proposes a novel interpretive theory of Locke's philosophy of mind that is structured around four neglected topics in his Essay Concerning Human Understanding: (i) his epistemic humility, (ii) his nominal dualism, (iii) his mind-body functionalism, and (iv) his naturalistic approach to the human mind. The book also explores the relevance of these overlooked views to contemporary debates in philosophy of mind, including Donald Davidson's anomalous monism, David Lewis' Ramseyan Humility, and Colin McGinn's cognitive closure. Locke's Ideas of Mind and Body will appeal to Locke scholars as well as anyone interested in early modern philosophy of mind and the history of the mind-body problem.