Yin and Yang are represented graphically by the tai ji symbol, shown below. In it, two separate but equal halves— black for Yin, white for Yang—swirl around each other and transform into one another, with each containing a kernel of the other at heart. TCM theory states that Yin and Yang are inseparable in life—we cannot have night without day, cold without hot, movement without stillness, etc—and when each remains in balance with the other, harmony prevails and we remain healthy. In human terms, Yin and Yang separate only at the moment we die, and even then the universal balance is maintained. All things remain in flux, yet and all things remain the same. (The theory of Yin and Yang applies to much more than just our health, but for the sake of this forum, we'll focus on its medicinal applications.)

These analogies are also seen in the homeostatic functioning of our bodies. Fire represents the flame that keeps us alive and stokes all our metabolic processes. Water in turn represents the cooling, nourishing aspects of our systems. Kidney Fire (in the form of Yang Qi) warms the Spleen, which in turn is able to transform our food into useable Food Qi. (Without that Yang Fire, our digestive system would be like using a rice-cooker that's not plugged in … you can wait all day, but the hard rice you have in the pot isn't going to cook.) The Lungs receive the "mist" from the cook-pot Spleen and in turn rain that mist down to the Kidneys, which are thus kept cool (tempering the Yang Fire). Likewise, the Fire of the Heart assists in our mental and emotional functioning, but, lest we get too "hot headed," our Kidney Yin keeps the Heart Fire in check. Yin and Yang thus maintain harmony and our bodies remain healthy.

Yin and Yang in TCM Treatment
Yin and Yang become imbalanced in a variety of ways, sometimes with a true excess or deficiency of one over to the other, other times with a relative excess or deficiency. One example of a true excess of Yang over Yin is a raging fever (redness and heat belong to Yang) from an acute flu or infection in an otherwise healthy person. The person's store of nourishing, cooling Yin fluids has not yet been damaged by the high fever, which has just begun; thus, the overall state of the Yin remains unchanged although the Yang blazes (normal Yin, excessive Yang). In this case, the treatment principle would likely be to sedate the Yang, rather than to nourish the Yin. The goal of treatment would be to bring the person back to equilibrium before strengthening their immune system to prevent future attacks.

An example of a relative excess of Yang over Yin is the low-grade fever and night sweats associated with menopause. Over the course of a woman's life, she may consume her Yin in ways both natural (childbearing, menstrual bleeding) and unnatural (relating to her lifestyle, such as smoking, over-consumption of spicy foods, not getting enough sleep). In this state, even if her Yang is at normal levels, her deficient Yin will show up as Heat, or a relative excess of Yang. In this case, the treatment principle would likely be to nourish the Yin, which is deficient, rather than sedating the Yang, which remains at normal levels.

Yin and Yang Separating
If the first patient's fever were allowed to burn uncontrolled, Yang would eventually overpower and consume Yin. Under the most extreme conditions, when Yang is allowed to entirely consume Yin, death (in this case by dehydration) will result, as one cannot exist without the other. The same would also prove true in the case of prolonged exposure to extreme cold, where Yin (cold weather) would eventually overpower Yang (a body's natural warmth) and lead to their "separation," death.