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?

One might think that in order to discuss the question of whether realism is plausible or reasonable, one should already know what it is. As philosophers know only too well, however, one cannot think about everything at once, and the debate surrounding realism is no exception. One must often assume coherent accounts of various components of a position in order to give careful attention to others, and people on all sides of this debate usually take a great deal for granted so as to focus on epistemic questions. For example, when describing their positions realists often rely heavily on things such as causation, laws of nature, and the natural kind structure of the world.