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The *Dongtian Yulu* (Record of the Caves and Springs) states: When Gao Shang Yuan Jun (High Supreme Prime Sovereign) first propagated the Great Dao in the mortal world, he drew analogies from the appropriate laws of heaven and earth's ascending and descending, and the numerical cycles of the sun and moon's往来 (intercourse), calling it Neidan (Internal Alchemy). Guangcheng Zi taught the Yellow Emperor to practice according to these methods, but despite persistent effort, no efficacy was seen. Thus, in the Kongtong Mountains, relying on internal affairs as the principle and combining minerals and metals, he called it Waidan (External Alchemy). The alchemical pills within are true medicine; those without can only treat diseases and bring about peace and comfort. Neidan, on a small scale, leads to longevity and freedom from aging; on a large scale, it allows one to transcend the ordinary and enter sainthood. It begins with the interaction of two qi (vital energies), condensing in the Dantian (Elixir Field), transforming Jing (Essence) into Mercury, Mercury into Cinnabar grain, and Cinnabar grain into the Elixir. Its shape resembles a bullet, its color like a vermilion tangerine, and true qi arises spontaneously. Refining qi with qi, qi and Shen (Spirit) gather to enter the Dao; when the Dao is accomplished, one enters sainthood. The sage becomes vast and transformative, penetrating all things—how could this be compared to Waidan? Therefore, knowing that true medicine and true elixir exist, there is no need to seek them outside the body.

The *Xishan Ji* (Record of Western Mountain) states: By the coupling of Dragon and Tiger, the Mysterious Pearl is formed; with precise Huohou (Fire Timing), the Great Medicine is refined. When Zhenqi (True Qi) begins to arise, elevating it can refine the form, avoiding the perils of cold and heat. Once Zhenqi has gathered, containing it can return it to the Elixir, permanently eliminating the suffering of hunger and thirst. Is this not medicine for oneself? When Zhenqi is applied to others, there are also verifications. Entering water causes it to boil; blowing on trees makes them flourish; bitterness turns to sweetness; decline transforms into strength. Using it to treat illnesses, there is no case that does not recover. Concentrating Shen to dispel demons, there is no instance that is not effective. This is because the internal truth corresponds externally—is this not what is meant by "medicine of man"? Some, failing to cultivate internal affairs and unaware of their True Yuan (Original Essence), rely solely on impure minerals and metals, adding sunlight and fire, ingesting them seeking transcendence and ascension; this is an error! However, superior scholars of ancient and modern times also discuss Waidan, asserting that Waidan is not unusable. For example, Guangcheng Zi used Zhusha (Cinnabar) as the elixir, refining it through nine transformations to call it Shendan (Divine Elixir). Chen Qizi used Seven-Treasure Cinnabar, refining it through nine transformations to call it Lingdan (Spiritual Elixir). Liu An Wang used urine from young boys, refining it through seven transformations to call it Huan Dan (Returning Elixir). From this we know that Ling (Spiritual Energy) gathers to become Shen; Shen scatters to become Void. Returning Yuan through Qi is called Huan Dan. Later generations using it have also seen efficacy. Those who ascend to immortality generally began by refining medicines while simultaneously cultivating internal affairs, achieving both internal and external perfection, thereby gaining access to the path of immortality. If Waidan medicine is used alone, Qi becomes weak and Shen declines; the exquisite Qi of heaven and earth cannot remain, causing harm that is not insignificant.