This begins as a book about scientific realism. To a very rough, first approximation, realism is the view that our best scientific theories correctly describe both observable and unobservable parts of the world. When philosophers consider this idea they are usually concerned to address the issue of whether it is a reasonable view to hold. They worry about whether it gives a plausible account of scientific knowledge, and rightly so! This is an undeniably important question. It is close to the heart of almost all issues in the philosophy of science, and importantly relevant to many issues in philosophy and the sciences more generally. This book, however, starts with a much more basic and arguably prior question. What is scientific realism, exactly?