In his Zhonghe ji (The Harmony of the Center: An Anthology), Li Daochun (late 13th century) makes an important distinction between two Elixirs, which correspond to different degrees of realization. The first Elixir is the Internal Medicine (neiyao), and the second one is the External Medicine (waiyao). Li Daochun says:
The External Medicine allows you to cure illnesses, and to prolong life and have lasting presence. The Internal Medicine allows you to transcend the world, and to exit from Being and enter Non-Being.The chart reproduced above contains a graphic summary of the two Medicines.
Reading from right to left, the first lines of the chart show the ingredients of the Internal Medicine: Precelestial Perfect Essence, Void Breath (Qi) of Empty Non-Being, and Indestructible Original Spirit. These are the precelestial Essence, Breath, and Spirit, which are already perfected of their own.
The next lines show the ingredients of the External Medicine, which Li Daochun calls Essence of the intercourse, Breath of breathing, and Spirit of the thinking mind. These are the coarse, postcelestial Essence, Breath, and Spirit, which are refined through the Neidan practice.
Then the chart shows the three stages of the Neidan practice, which Li Daochun calls the "three barriers", and which lead from "doing" to "non-doing":
(1) Refining Essence to transmute it into Breath. This stage is characterized by "doing". When it is represented by the trigrams of the Book of Changes, its purpose consists in taking the inner line of Kan ☵ (this line represents True Yang) and returning it to Li ☲, to which it originally belongs.
(2) Refining Breath to transmute it into Spirit. This stage is characterized by the "merging of doing and non-doing". Its purpose is conjoining Qian ☰ and Kun ☷, which represent True Yang and True Yin, respectively.
(3) Refining Spirit to return to Emptiness. This stage is characterized by "non-doing". Its purpose is reverting from Pure Yang to Emptiness. Pure Yang, also represented by Qian ☰, is the state of Unity, before the division of the Great Ultimate (taiji) into Yin and Yang.
Attaining the Internal and the External Medicines, says Li Daochun, depends on one’s individual qualities:
In general, those who study the Dao should begin from the External Medicine; then they will know the Internal Medicine by themselves. Superior persons have already planted the root of virtue, and know it by birth; therefore they do not refine the External Medicine, and directly refine the Internal Medicine.Therefore, according to Li Daochun, "superior persons" innately possess the Internal Medicine and are immediately able to "transcend the world". Everyone else should begin from the External Medicine, through which they can reach the point in which they “will know the Internal Medicine by themselves,” and attain the same degree of realization as those who innately possess it. This gradual process is the purpose of Neidan.