What is the principle of self-repair of the physical body?

The physical body possesses a self-repairing function: when the body is damaged, it automatically mends itself; when internal organs fail, they automatically repair; when the body is exhausted, resting for a period restores its energy; after surgery, the incision heals on its own; missing muscle, skin, and so forth are automatically replenished. When blood is lost, it can be regenerated; when there is no food or drink in the stomach and intestines, the body can automatically replenish energy—the practice of bigu (grain avoidance) to regulate the physical body makes use of this marvelous mechanism. What mechanisms are all of these? Whose functions and operations are they? When did this marvelous source of life first appear?

Master Kuiji, in a previous life, entered meditative concentration and waited several thousand years for Śākyamuni Buddha to appear in the world, but as a result missed Him while in samādhi, and so wished to enter concentration again to wait for Maitreya Buddha to appear in the world. A human body that relies on the four great elements of food and drink to sustain life—yet for thousands, even tens of thousands of years in concentration, neither eating nor drinking, without the slightest discomfort, and able to remain in concentration for tens of millions of years without dying—by what mechanism did Master Kuiji and others who attained longevity through concentration sustain their lives? There is said to be an experiment showing that thin people live longer than fat people, and those who eat less live longer than those with big appetites—what is the principle behind this?

Modern medical health theory holds that one must sleep between the midnight hours of eleven o'clock and one o'clock; otherwise, staying up late deprives the internal organs of rest and repair, and the body will become depleted. Yet sitting in meditation and entering concentration at midnight causes no harm. Back in the day, the Buddha encouraged his disciples to practice diligently, exerting effort unremittingly through the first watch, middle watch, and last watch of the night, and no one ever heard of the bhikṣus suffering any physical discomfort; on the contrary, their energy was more vigorous and their minds more acute in thinking. Modern medicine also says that skipping breakfast makes one prone to gallstones; yet in the Buddha's time, the monastics ate only one meal a day at midday, and no one ever heard of any of them getting gallstones. By what mechanisms do all of these operate?

When you sun yourself, the body automatically replenishes yang energy and vitality; if you can contemplate with single-minded visualization, the replenishment will be greater and the body regulated even better. Apart from the devas of the formless realm, who have no bodies and do not need the nourishment of the four great material elements, all sentient beings with bodies need the nourishment of the four great elements. The higher the realm of a sentient being, the fewer the four great elements it needs, and the more subtle and refined those materials are. Sentient beings in lower realms, due to meager merit (alpa-puṇya), must partake of extremely coarse and inferior four great elements. Humans are in the middle stratum of the desire realm, not among the most advanced beings of the desire realm; the four great elements they partake of are rather coarse and inferior, yet they believe that what they enjoy are high-grade substances and superior food and drink—when in reality these are very coarse. Only the devas of the form realm partake of the highest-grade, most refined four great elements; next come the devas of the desire realm, and after them the immortals. All humans are inferior to the devas in what they enjoy: their food is not only coarse but also extremely filthy, with poor color, aroma, and taste. Yet because their karmic conditions are such, they do not feel anything is wrong; on the contrary, they are deeply attached to it—just as dogs delight in dung and worms delight in cesspits.

All the four great elements partaken of by the repair mechanisms of every physical body are the functions and operations of the tathāgatagarbha, created by the tathāgatagarbha in accordance with sentient beings' karmic seeds and merit. Because sentient beings have karmic connections with the human world of the desire realm, possessing the karmic seeds and karmic conditions to be humans or animals, if the five-aggregate body is to survive normally in the desire realm, the physical body must be endowed with a sufficient supply of nutrients from the four great elements. If the four great elements in the physical body are deficient, this will affect the normal functioning of the five-aggregate body, and the tathāgatagarbha will, according to karmic seeds and karmic conditions, automatically replenish the missing four great elements. If karmic obstructions do not permit replenishment, then the five-aggregate body continues to function imperfectly with an incomplete physical body—this reveals the evil karmic conditions of sentient beings. For sentient beings endowed with wholesome karma, whatever the physical body lacks, the tathāgatagarbha will automatically replenish; if it cannot be replenished, that is the evil karmic retribution of a physically incomplete and impaired body.