Time goes by, and then you study, but it's hard to work hard. This paper explains the principle of unbalanced development of individual body and mind in pedagogy.

This language comes from Xue Ji, the original text is: "When it can be said, when it is too difficult to learn." It means: don't go to school until the appropriate study period is over. Although you study hard, you have accomplished nothing. This reflects the imbalance of people's physical and mental development process, which has a critical period. In the critical period, the effect of cultivating students' corresponding ability is the best; After the critical period, even if you try harder, the effect will not be good.