Thought experiments are performed in the laboratory of the mind. Beyond this metaphor it is difficult to say just what these remarkable devices for investigating nature are or how they work. Though most scientists and philosophers would admit their great importance, there has been very little serious study of them. This volume is the first book-length investigation of thought experiments. Starting with Galileo's argument on falling bodies, Brown describes numerous examples of the most influential thought experiments from the history of science. Following this introduction to the subject, some substantial and provocative claims are made, the principle being that some thought experiments should be understood in the same way that platonists understand mathematical activity: as an intellectual grasp of an independently existing abstract realm. With its clarity of style and structure, will find readers among all philosophers of science as well as scientists who have puzzled over how thought experiments work.