From their search for the laws of the universe the Taoists discovered within the apparent chaos of nature a predictable and rhythmic cycle - Yin & Yang. Because The law of Yin & Yang can be applied to anything in existence it is a very powerful tool of analysis and understanding. There are five points to the law of Yin & Yang.
1) OPPOSITION
Everything has two opposite aspects. Yin & Yang struggle with and control each other.
2) INTER-DEPENDENCE
Yin & Yang define each other and therefore one cannot exist without the other. (If there is no down, which way is up?)
3) MUTUAL CONSUMPTION & SUPPORT
Yin & Yang each give of themselves to nourish the other.
4) INTER-TRANSFORMATION
YIn can become Yang and Yang can become Yin. In fact, this is inevitable if the growth of one or the other is uncontrolled.
5) INFINITE SUBDIVISIBLITY
There is always a bit of one in the other. Anything can be subdivided again and again.

EXAMPLES OF YIN YANG RELATIONSHIPS

• OPPOSITION - ocean and beach, Democrat and Republican
• INTER-DEPENDENCE - up and down, in and out
• MUTUAL CONSUMPTION & SUPPORT - cows and pasture, tree and moss
• INTER-TRANSFORMATION - malaria, economic boom and bust
• INFINITE SUBDIVISIBLITY - United States Tax Code

The Yin Yang relationships below are utilized so frequently that each of the characteristics has been designated either Yin or Yang. Fixed categorization would violate the laws of Yin & Yang but the designations aren't fixed. They're simply a convenience which simplifies communication among those familiar with Yin & Yang.

Yin & Yang are so entwined that one cannot exist without the other. They are irrevocably two parts of a whole. That’s why their relationships are called The Law of the Unity of Opposites. When forces of the universe create a Yin and a Yang they come together and, in the case of humans, create a life. As long as Yin and Yang maintain their dynamic oscillating balance the life exists. The closer to the center of their oscillation the extremes of their cycles are, the less illness will occur. When they separate, that life ceases. These facts yield a very useful definition: Health is life in balance.
Yin & Yang are so entwined that one cannot exist without the other. They are irrevocably two parts of a whole. That’s why their relationships are called The Law of the Unity of Opposites. When forces of the universe create a Yin and a Yang they come together and, in the case of humans, create a life. As long as Yin and Yang maintain their dynamic oscillating balance the life exists. The closer to the center of their oscillation the extremes of their cycles are, the less illness will occur. When they separate, that life ceases. These facts yield a very useful definition: Health is life in balance.
You may be wondering what all this has to do with alleviating human illness. Human anatomy, physiology, psyche, and environment are analyzed according to which characteristics of Yin and Yang each possesses. This chart gives characteristics which are often applied to people.

The result of the analysis describes what is wrong with the patient (a diagnosis) and that diagnostic description determines treament principles and strategies
Let’s look at a very simplified example. A 35 year old female complains of having

1. no energy. When not working she sleeps most of the time.
2. She feels cold often and is uncomfortably sensitive to cool environments.
3. Her skin appears damp and damp foods make her sleepy.
4. She feels dull emotionally and intellectually.
5. Her posture is poor with shoulders that slope forward giving the appearance of a caved-in chest.
6. When she speaks only her lips move.
7. She has been having less contact with her friends, preferring to be alone. DOES SHE FIT YIN OR YANG?

Here is a classification of her symptoms by Yin & Yang characteristics. You can check them against the list of characteristics you just read.
1) stillness 2) cool/cold 3) moist 4) passive
5) inward 6) stillness 7) inward