General Introduction -- The philosophy of science is at least as old as Aristotle, but it has risen to special prominence in the twentieth century. As scientists have made tremendous advances in fields as diverse as genetics, geology, and quantum mechanics, increasing numbers of philosophers have made science their focus of study. In its broadest terms, the philosophy of science is the investigation of philosophical questions that arise from reflecting on science. What makes these questions philosophical is their generality, their fundamental character, and their resistance to solution by empirical disciplines such as history, sociology, and psychology.