Principles of Mathematics/Introduction/Why I write this book
Why I write this book 
This book is a simpler and easier-to-understand Principia Mathematica, written by Alfred North Whitehead and Bertrand Russell, published in 1910, 1912 and 1913. I suffered a lot of pain while reading that book, so I make an easier version, based on my understanding. I think it will be helpful.
When I was young, they teach me 1 + 1 = 2. I asked them "Why?". They just laughed at me and said "1 + 1 is 2, that's all!". If you also laugh, then this book is not for you. This book is intended to ones who are really serious about reasoning, not for ones only interested in common practice.
1 + 1 = 2, it's not that simple. It is analogous to language. We at least speak one natively, but no one has been able to tell a general rule about how it works. The most basic things are the hardest to explain.
This book will guide you through the fundamentals of mathematics. It does not start from counting apples. It does not start from learning multiplication tables. It starts from logic, and logic will be our king, friend and tool throughout this book.