People's natural life span, called "natural year" by the ancients, is considered to be related to innate vitality. Of course, people's life span is limited and often depends on the strength of vitality. From this point of view, it is obvious that people are in a passive position, so Zhang's "Pure Moon Book" says: "This person's life is in heaven." However, on the other hand, few people can do their best, which has a lot to do with whether they can take good care of their bodies the day after tomorrow: although they are born strong, they can still die without scruple; Although born weak, diligence and prudence sometimes lead to longevity. First of all, Zhang is not only a congenital theorist, but also attaches great importance to the role of acquired. As he said in the theory of nature and nurture: "Those who cultivate the day after tomorrow live long, and those who pray for cutting the day after tomorrow die even worse." "If people (the day after tomorrow) play a role, they will not be able to rely on their innate strength, and they will lose their strength if they rely on it; The innate weak should be cautious, and caution can overcome the sky. " "On Zhongxing" also pointed out: "If the Tao is obtained, the counselor has the right and the manpower is in the majority." From this perspective, people have certain initiative in their own lives to a certain extent. Therefore, Zhang also said: "The life of this day is also the life of people." Through this analysis, Zhang highlighted the dynamic role of people in mastering their own lives and reached the conclusion that "the day after tomorrow is people", which is really convincing.