"Scholars must be suspicious first." -Cheng Yi "Those who doubt but don't doubt have never learned; Knowledge is in doubt. " -Zhang Zai's understanding is based on facts and evidence. There are two sources of facts and basis: one is what I saw with my own eyes, and the other is what I heard from others. For example, when the country is in danger, there must be a lot of oral news from all over the country, no matter how dangerous, it is the legend of others and not necessarily reliable; To understand the actual situation, only by yourself. The same is true of learning, the most important and reliable material is the factual basis that I have seen with my own eyes; But sometimes such evidence can't be seen with your own eyes, and you can only rely on other people's legends. We should think about the legendary words, believe it or not, and should not believe it casually. We believe it because it is; Don't believe it, because it's not. This attitude of thinking ahead and not believing casually is the spirit of doubt. This is the basic condition for doing all learning. We heard that there were three emperors and five emperors in ancient China, so we have to ask: Who said that? In which book did you first read it? Who wrote the book you saw when? How did the author know? We have also heard that "rotten grass turns into fireflies", so we have to ask: How can dead plants turn into flying beetles? What is the scientific basis? If we can ask this question, all wrong theories will be broken. No matter what kind of books and knowledge we have, we must experience our own doubts: think because of doubts and distinguish right from wrong because of thinking. After three steps of "doubt", "thinking" and "discrimination", that book is your own book, and that kind of learning is your own knowledge. Otherwise, it is blind superstition. Mencius said that "it is better to believe in books than to have no books", that is, to teach us to have a little skepticism, not to follow blindly and not to be superstitious.