An introduction to the concept of the book, the role it aims to play, and the intended audience. Traditionally, mathematics is often thought to progress through levels of difficulty, with category theory being very advanced and therefore only taught to advanced math students. However, in this book we take the view that mathematics is a network of interconnecting ideas, and that, while individual subject areas may progress cumulatively, there is no technical need to study any of them before category theory. So the book is aimed at a wide audience, especially including people who have not studied college-level mathematics.