This book argues for a new image of nature and of science—one that understands both natural and social phenomena to be the product of mechanisms, and that suggests that much of the work of natural and social scientists involves discovering, describing, and explaining how these mechanisms work. The book explores the interplay between ontological questions about mechanisms as things in the world and methodological questions about how these mechanisms can be characterized. Ontologically, mechanisms are understood to be collections of entities whose organized activities and interactions give rise to phenomena. This minimal conception of mechanism is abstract enough to encompass most of the wide variety of things that scientists have called mechanisms. While mechanisms are particular things, localized in space and time, the models that scientists use to describe them must be abstract and idealized. The mechanistic approach provides new ways of thinking about traditional metaphysical questions—for instance, about the nature of objects, part-whole and cause-effect relations, properties and universals, natural kinds, and laws of nature. It also suggests novel approaches for thinking about methodological questions concerning scientific representation, causal inference, reduction, and scientific explanation. The New Mechanical Philosophy offers the promise of a better understanding of the sources of both the unity and diversity of science.