Since the Taoists believe that everything is part of the oneness of the universe, they make no distinction between the external forces of nature and the internal processes of the human body, believing that "the macrocosm exists within the microcosm." In other words, any process or change that can be witnessed in nature can also be seen in the body.

For example, a person who eats cold food (yin) on a cold, damp day (yin) may experience excessive mucus (yin). Similarly, a person who performs strenuous activity (yang) on a hot day (yang) might experience dehydration with a fever (yang). Some of the traditional diagnoses sound like weather reports, such as "wind and cold with dampness" (a yin condition) or a "deficiency of moisture leading to fire" (a yang pattern).

These diagnostic descriptions illustrate the principle that the body experiences the same fluctuations of yin and yang as the environment. The internal organs also have their own balance of yin and yang.

Yin functions tend to be nourishing, cooling, building, and relaxing and relate to the structure, or substance, of the organs. Yang qualities tend to be energizing, warming, consuming, and stimulating and relate to the functional activity of the organs.

For example, the kidneys are considered the source of yin (water) and yang (fire, or metabolism) for the entire body. If the kidney yin is deficient or depleted, a person can experience hot flashes and night sweats, as occurs in menopause when estrogen (yin) levels decline. This is due to insufficient moisture (yin) to keep the metabolic fire (yang), which keeps the body warm, under control.

Think of running a car engine with insufficient oil; the engine will overheat due to a deficiency of this yin-like lubricant. A deficiency of kidney yang produces such symptoms as cold hands and feet and a general lack of vitality.

These symptoms, which often occur with age, are due to insufficient metabolic fire (yang) to infuse the entire body with energy and warmth, dispersing cold and fatigue (yin).