Make friends with the same interests, learn from each other and encourage each other in character. Specifically, "fen" refers to congenial feelings, that is, having the same interests and pursuits; "Cutting and grinding" means learning from each other and making progress together; "Proverbs" are telling each other their own conduct and reminding each other of their mistakes. This sentence emphasizes the importance of making friends and points out the correct way to get along with friends. The principle of "friendship" in the five ethics is an extension of this concept and the principle of brotherhood. In this way, we can study and make progress with our friends and establish good friendship.