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Between heaven and earth, all things are nurtured and matured; when male and female combine their essence, all things are born. This is the natural law of creation, a way of non-thought and non-action. Therefore, where there is human conduct, there are husbands and wives; where there are husbands and wives, there are offspring. How can there be talk of lacking heirs? However, there are times when heaven does not generate, and regions where earth does not sprout vegetation; thus, humans cannot be without those who lack heirs. If so, those who bear children do so naturally, and those who lack heirs remain so. Then why speak of seeking heirs? Can it truly be sought? Can it truly not be sought? There are indeed principles and methods within this. The principle discussed here is not for those congenitally unable to conceive or barren lands that yield no crops, nor for those in their prime with robust vitality. Congenital infertility stems from innate endowment, which cannot be altered by human effort; robust vitality in youth arises from the natural harmony of yin and yang, requiring no conscious knowledge or deliberate action. It applies only to those capable of having children but have not, who find childlessness unbearable, as this is not necessarily heaven's decree; and to the elderly who lack sons, whose energy wanes daily, unlike the young. Thus, there is room for human intervention, providing both rationale and method. Although ancient methods are numerous, I feel they do not fully capture their subtleties. Therefore, I list these methods: Heavenly Timing (Shi Tian), Earthly Advantage (Di Li), Human Affairs (Ren Shi), Medicinal and Dietary Therapy (Yao Shi), and Diseases (Ji Bing), totaling five categories and twenty-four items. Neglecting any one of them can ruin the endeavor. I had a son in my old age, relying on these principles. Those suffering from infertility should observe, believe, and apply them, and the plan for continuing the lineage may not be insignificant. Hence, the title *Yilin Ce* (Strategy for Seeking Heirs).

Regarding the timing of intercourse for conception, ancients advised selecting auspicious days and hours, such as those blessed by the Heavenly Virtue (Tiande) and Monthly Virtue (Yuede), or when heavenly stems and earthly branches are prosperous, while avoiding days associated with Bing and Ding (Fire elements). However, in moments of urgency, how can one wait to select an auspicious time before acting? Such advice seems pedantic and impractical. Nevertheless, conditions such as clear sunny weather, gentle breezes, mild and refreshing climate, along with a calm mind and relaxed spirit, are universally recognizable without needing specific selection. Conception under such favorable conditions results in offspring who are not only healthy but also intelligent and virtuous. The constitution of the fetus is fundamentally established here. Conversely, those ignorant of taboos may encounter the dimness of heaven and earth, absorbing qi of stupidity and confusion; encounter eclipses of sun, moon, and stars, absorbing qi of defect and conflict; encounter violent thunderstorms, absorbing qi of ferocity and madness; or encounter erratic shifts between hot and cold, absorbing qi of deceit and treachery. When cosmic qi is abundant, aligning with it brings longevity; when it is contracting, violating it leads to premature death. Humans live within the six directions, and every stage from growth to decline is shaped by the qi received from creation. Intelligence, foolishness, virtue, and vice are all determined by the constitution endowed by heaven and earth. This is the fundamental root of sensing the primordial origin. To alter one's destiny and assist in the nurturing of life, this must be the primary concern.