Within Zhou Dunyi’s Taiji philosophy, qi is the vital, dynamic substance through which the Great Ultimate unfolds into the world of concrete things. Taiji itself is understood as a metaphysical, moral–cosmic principle that does not appear directly; it requires qi as its material and energetic medium. In this sense, qi is both the “stuff” and the activity that allows the patterns of Taiji to be embodied in the realm of phenomena. It is the substantial basis of existence, yet always in motion, always responsive to the overarching principle.

Qi also provides the field in which yin and yang arise and interact. When Taiji moves, it generates yang; when it becomes still, it generates yin, and these are not merely abstract poles but distinct modes of qi—more dispersed as yang, more condensed as yin. From the ceaseless alternation and mutual influence of yin–yang qi, the Five Phases (wood, fire, earth, metal, water) emerge as structured configurations or phases of this same underlying qi. In this way, qi serves as the vehicle by which the most subtle cosmological principle differentiates into the rich variety of cosmic processes.

Through the condensation, dispersion, and patterned configuration of qi, heaven, earth, and the “ten-thousand things” come into being. All entities share the same fundamental qi-nature, yet differ because their qi may be clearer or more turbid, more balanced or more disordered. This shared substrate allows for an underlying unity amid the diversity of forms, while still accounting for the evident differences in quality and function among beings. Qi thus maintains continuity between the highest cosmological order and the most ordinary things.

In human life, qi is at once material endowment and vital force, shaping temperament and character as the concrete expression of an underlying nature grounded in principle. Moral and spiritual cultivation, in this framework, consists in rectifying and harmonizing one’s qi—through sincerity, reverence, and inner calm—so that the pure pattern of Taiji can shine through more fully in conduct. By refining qi, the human being aligns more closely with the cosmic process, allowing the same energy that generates heaven and earth to manifest as clarity, balance, and virtue in the heart–mind.