This book aims to explain social phenomena. Many of its methods, theories, and examples come from economics, but this book is not entirely about economics. If it were purely an economics book, it would have to use well-established economic theories to describe and analyze specific research objects, and it must discuss concepts and principles such as "supply", "demand", "price", "utility", "marginal", etc. Otherwise, it would be irresponsible to the reader. This book strives to explain various social phenomena using scientific methods. Of course, economics is also a science, but economics has already developed a fixed research procedure. Here, we just want to reveal to the readers what lies behind many phenomena. Some parts of this book are "philosophical", while others are "logical". What I want to say is that there are profound "scientific" reasons behind the social phenomena we see - I believe that the essence behind these phenomena is rationality and logic. If this book can provide some enlightenment to the readers, I will be satisfied. As long as the readers feel that they have gained more than the price they paid for this book, my conscience will be comforted, because I hope that writing this book and the readers buying it constitute a "cooperative game" rather than a "non-cooperative game."