Chinese medical theory holds that jing, qi and shen form the Three Treasures, sanbao, a hierarchy of increasingly refined substance and function, with jing as the densest and most foundational of the three. Prenatal jing, xiantian zhi jing, is inherited once, at conception, from the combined essence of one's parents, is fixed in total quantity, and is slowly spent over the course of a lifetime; postnatal jing, houtian zhi jing, is continually extracted from food and air and can replenish the system day to day, though it cannot restore what prenatal jing has lost.
Both are stored chiefly in the kidneys, which the Huangdi Neijing and later texts treat as the root organ of growth, reproduction, bone and marrow, and hearing, so that kidney-jing deficiency is diagnosed behind conditions of infertility, weak bones, premature graying and developmental delay in children. Daoist internal alchemy, neidan, builds its entire practice on this hierarchy, describing cultivation as a three-stage refinement: jing is refined into qi, qi is refined into shen, and shen is finally returned to emptiness or the Dao, a sequence that makes conserving jing, especially through moderating sexual activity, the necessary first step before the higher refinements can proceed.