“The qi(vital force) of heaven and earth” denotes the two types of qi, an element that constitutes all things corporeal and flows between heaven and earth. Both shapeless, the qi of earth and the qi of heaven respectively possess the opposite attributes of yin and yang. Their movement follows the law of the change of yin and yang. The interaction between the two types of qi leads to the day-and-night cycle, the succession of the seasons, and the occurrence of such heavenly phenomena as wind, rain, thunder, and lightning. The interaction also determines how all the things are generated and changed.

The qi of heaven and that of earth merge into one thing; it evolves into yin and yang, the four seasons, and the five elements of metal, wood, water, fire, and earth. (Dong Zhongshu: Luxuriant Gems of The Spring and Autumn Annals)

The change in the qi of heaven and that of earth prioritizes harmonization, which refers to the reconciliation between yin and yang, the differentiation between day and night, and then the generation of all the things. (Huainanzi)