"Tian Yin Zi". The original title is "Shu" by Sima Chengzhen, and it was written around the end of the Northern and Southern Dynasties to the early Tang Dynasty.
There are eight chapters in total, and the book is divided into the first one for Immortals, the second for Yi Jian, the third for Jianmen, the fourth for fasting, the fifth for peace, the sixth for thinking, the seventh for sitting and forgetting, and the eighth for divine understanding.
It tells that the way of immortality is based on immortality, and nourishing qi is the first step in achieving immortality; qi is received by heaven and earth, and harmonized with yin and yang; yin and yang are called the heart, and the heart governs day and night, which is the soul; people can obtain the immortal way through hard practice.
Source at the bottom: "Orthodox Taoist Canon" Taixuan Bu.
