Second, the vital essence at the time of conception from one's parents is known as 'Prenatal Essence Qi.'
Third is the 'Essence Qi of Grain and Water,' generated from food to sustain human life. It is also called 'Postnatal Essence Qi.'
The mysteries of the first two were elusive and beyond control, even for someone like Zhang Beixing. He could not yet put the core ideas of the Heaven and Earth Natural Technique into practice. Therefore, one must start with 'Postnatal Essence Qi.'
People researched it. During the Han Dynasty, someone proposed the concept that "Earth dominates the seasons, patterns interweave, green, red, white, and black each have their place, all governed by the central palace, the function of Wu and Ji." This concept was subsequently recorded in the *Zhou Yi Can Tong Qi*. This laid the foundation for the cultivation of 'Postnatal Essence Qi.'
