How fixed are the happenings in Nature and how are they fixed? One - very orthodox - account teaches that the sciences offer general truths that we combine with local facts to derive our expectations about what will happen, either naturally or when we build a device to design, be it a laser, a washing machine, an anti-malarial bed net, or an auction for the airwavse. Nancy Cartwright offers a different picture, one in which neither we nor Nature have such nice rules to go by. This volume includes her Paul Carus Lectures plus four additional papers that further develop some of the central ideas. Cartwright offers an original view on the age-old question of scientific realism in which our knowledge is genuine, yet our scientific principles are neither true nor false but are, rather, templates for building good models.