"In ancient times the holy sages made the Book of Changes thus:
Their purpose was to follow the order of their nature and fate.
Therefore they determined the tao of Heaven and called it the dark and the light.
They determined the tao of Earth and called it the yielding and firm.
They determined the tao of Man and called it benevolence and righteousness.
Counting that which is going into the past depends on the forward movement. Knowing that which is to come depends on the backward movement. " [W/B]
The holy sages left behind in the Yijing much important information on the secrets to Tao and the Changes for posterity.
Not many down the ages grasped and understood the secrets. Those who did and went on to cultivate their nature (Xing) and fate (Ming) in line with the secrets left behind in the Book of Changes (I Ching / Yijing) with the successful becoming sages (Sheng Ren), or Zhen Ren (realized persons) and Daoist celestial immortals.
These ancient sages and Zhen Ren wrote down their understanding of Tao and the Yi in their texts and classics to help sincere and earnest students. Yet there remained a gap of about seven centuries (from 300 AD to 1,000 AD) where the ignorant and the gullible concentrated on outer alchemy (Waidan) instead of inner alchemy (Neidan) to cultivate Tao to achieve immortality.
About a millennium ago, Chen Tuan and Chou Dunyi helped correct the thoughts of Daoists and Confucians with the publication of their respective Wujitu (Map of the Infinite) and the Taijitu (Map of the Supreme Ultimate).
In his treatise on the Taijitu, Chou Dunyi had mentioned most of the highlighted keys to Changes and neidan. He also dealt briefly with Qi (breath). If Daoists and Buddhists choose to ignore his contributions because Chou happened to be a Neo Confucian, they may not know that he obtained the Taijitu from a student of Chen Tuan.
While Chou Dunyi may not be as deep as the Zhen Ren, by looking at the Taiji symbol, he certainly knew much about Change. When yin reaches its extreme, it will change to yang. When yang reaches its extreme, it will change to yin. Therefore the Taiji is represented by a circle half filled with yin and a touch of yang, while the other half is filled with yang and a touch of yin.
The explanation is in line with Yi divination whereby a mature yin line changes to yang and a mature yang line changes to yin. The young yin yang lines do not change.
But Changes are deeper than this, so is neidan.
Therefore we need to continue to study and learn, and cultivate our nature and fate. Without being empty and still, how could the esoteric ever hope to return to Destiny and the Infinite? (Refer to the Tao Te Ching chapters 16 and 28)