This collection of essays breaks new ground in bringing together scholars from a range of disciplines to focus on the nature and status of principles in early modern thought. A comprehensive introduction argues that there is a natural "fault line" between propositional and ontological principles, and establishes a clear understanding of how the term principle might be used, and of the kinds of questions that might be raised about its usage. With contributions from leading scholarsincluding Daniel Garber, William Newman, and Sophie Rouxthis book will be of interest to scholars of early modern philosophy, the history of early modern thought, and the history and philosophy of science.