Within the understanding of Qi as the vital energy permeating all life, yin and yang are regarded as its two fundamental, complementary expressions rather than as separate substances. Every manifestation of Qi contains both aspects in varying proportions, so that no phenomenon is purely yin or purely yang. Yin is associated with receptive, passive, cooling, inward, and conserving qualities, while yang is linked with active, dynamic, warming, outward, and expansive tendencies. These are not rigid categories but relational descriptions that help illuminate how Qi behaves and appears in different contexts.

The relationship between yin and yang is inherently dynamic and interdependent. Each depends on the other for definition and existence, and each contains the potential to transform into its opposite. Periods of rest and stillness (more yin) give rise to movement and activity (more yang), just as intense activity eventually necessitates restoration and quietude. This ceaseless alternation and mutual support form the rhythmic pattern through which Qi circulates, transforms, and sustains life.

From this perspective, harmony in body and cosmos arises when the yin and yang aspects of Qi are in appropriate balance and flowing freely. Excess or deficiency of either aspect, or stagnation in their interaction, is understood as disharmony, which may manifest as illness or dysfunction. Practices that work with Qi—such as those that regulate breath, movement, and awareness—are often oriented toward restoring this balance, encouraging neither extreme but a responsive, living equilibrium. In this way, yin and yang serve as primary organizing principles for contemplating how vital energy moves, changes, and maintains order within the ever-shifting fabric of existence.