Like Tao and the Book of Changes, the ancient method of the Holy Sages, aka Neidan (inner alchemy) nowadays, is also deep and profound.
Bear in mind that Laozi and the Zhen Ren in their respective writings have made references to the foremost ancient Chinese classic, The Book of Changes.
Knowing something about neidan does not mean that the practice will be correct.
If people think that neidan is only about meditation, then they have missed the mark. (Refer to the poem on what are required.)
As indicated by Laozi in the TTC, those without superior virtue should not start with neidan meditation. Illusions and delusions can arise.
If you really want to practice neidan correctly, study the four Books and the five Classics (which include the Book of Changes) in order to become learned and wise.
While becoming learned and wise, students could come across how to keep the heart still. And here are more reasons to becoming learned and wise:
The Golden Flower is the Elixir of Life (Jin Dan – Golden Pill). All changes of spiritual consciousness depend upon the heart. There is a secret charm which, although it works very accurately, is yet so fluid that it needs extreme intelligence and clarity, and the most complete absorption and tranquility. People without this highest degree of intelligence and understanding do not find the way to apply the charm; people without this utmost capacity for absorption and tranquility cannot keep fast hold of it.
[The Secret of the Golden Flower by Lu Dongbin and translated by Wilhelm/ Baynes]
Real teachers if found may not be able to teach the entire practice to students. Students would have to put in their own sincere efforts and there is much to study and learn.
If practitioners after several years or decades of practice still cannot witness or experience any of the various major signposts of the Way, then they are either not practising neidan at all, or doing it wrong; simple as that. This comment equally applies to the indolent directly taught by Daoist Celestial Immortals.
The ancient major signposts are embedded in the Book of Changes (the Zhouyi), the Tao Te Ching, and the various Daoist texts written by the Zhen Ren before or after they became Celestial Immortals. The Shurangama Sutra also contains a few of these signposts.
If you happen to be a disciple of a major Daoist sect, you can present the poem to the elders for further clarification. (This note is especially applicable to readers from Hong Kong, Malaysia, Singapore, and Indonesia.) The elders may either smile or frown after reading it. But do not ask the Celestial immortals to explain. You may get a knock on the head. The punishment is for not doing your homework after so many reminders over the years. Ha!
The temple elders would probably smile when they recognize that their cultivation is based on the same foundation as the ancient method of the Holy Sages.
They would probably frown for not having reached the higher levels of neidan practice also depicted in the poem.
How would I know?
The adepts say that sometimes they feel light (as if floating), the heat during meditation can scorch, some experiences are intense, and some very intense. On what type of experiences, they do not reveal.
Quanzhen Patriarch Lu Dongbin also mentioned in the Secret of the Golden Flower that confirmatory experiences are like drinking water. The one who drinks it would know whether the water is warm or cold.
Perhaps these two previous articles on a simple circle and the concept of duality respectively can further assist readers in understanding the poem: