The ancient Chinese considered harmony to be the goal of our lives as individuals, and also as a society. When harmony is disturbed in our physical or emotional lives, it is restored by the use of acupuncture. Harmony is a continual balancing act between opposites: opposites in color, in energy, in actions, etc. This idea of opposites is seen even in the west with such ideas as positive and negative ions in chemistry and physics. The Chinese denote the opposite ends of each idea as "yin" and "yang". They are not opposed to each other, but are the ultimate in each direction. For example, rest is Yin and exercise is Yang, or Yin is cold winter and Yang is hot summer. Life would not be in balance if it were entirely exercise or entirely rest, and so balance of Yin and Yang produces harmony. Notice that Yang is the active, warm, excitable, aggressive side, whereas the corresponding opposite Yin is restful, cool, calming, and passive.
Acupuncture is concerned with the optimal flow of the energy Qi. Harmony is disturbed by a lack of balance, and a lack of balance will constrict or overemphasize the flow of Qi at various points in the body. The acupuncture practitioner has four sets of diagnostic features, each of which have a yin end and a yang end. Three of these are specific: hot and cold, interior and exterior, and excess and deficiency. So, for example, someone who spends all of their time inside eating sweets has at least two imbalances that can be noted by the acupuncture practitioner. There is also a fourth, general set, for any other features that should be noted in the acupuncture diagnosis that are not covered by the first three: for example, an unusually passive person. A harmonious personality should have a balance between aggressiveness and passivity, each at the appropriate times.
It may be surprising that the treatment of acupuncture takes mental and emotional states into account, but the goal of acupuncture is to restore harmony to the whole person. A number of imbalances may not have caused a physical health problem, but rather such symptoms as strife within the family, a tendency to cry easily, or other social or emotional symptom. These are strong indicators and also need to be addressed.
The goal of is to restore harmony and redirect Qi energy to its normal flow. This energy is active and always moving, and hence has Yang qualities. So, if you were an acupuncture practitioner and had a client who noticed they where being unusually aggressive and angry lately, you would suspect a buildup in energy at some organ in the body. Whereas, if someone were depressed and listless, that would be an indication of a deficiency of energy at some organ or organs in the body. This, along with a diagnosis of physical symptoms, would give the best acupuncture treatment to address this.
So, by organizing objects, actions, conditions, and other attributes of life into "Yin" and "Yang", the acupuncture practitioner can more easily fit mental, social, and emotional issues into the overall treatment plan for each of the clients. The method of acupuncture is to restore the normal flow of Qi, which in turn will restore harmony and balance into the patient's life.