Emperor Taizong made it clear. The first sentence needs no explanation. You can dress up in front of the mirror. In the middle sentence, we can refer to history to sum up the law of the rise and fall of things, because many things we are doing now have actually happened before and can be used for reference; Comparing yourself with others, you can see your own successes and shortcomings. In short, these words warn people: correct attitude, pay attention to comparison and reflection.