Acupuncture and Moxibustion are both rooted in the Yin-Yang and the five elements. The theories of yin-yang and the five elements were two interpretations of natural phenomena that originated in ancient China. They reflected a primitive concept of materialism and dialectics and played an active role in promoting the development of natural science in China. Ancient physicians applied these two theories to the field of medicine, greatly influencing the formation and development of the theoretical system of traditional Chinese medicine, and guiding clinical work up to the present time.

Yin-Yangtra1) The opposition of yin and yang. The theory of yin-yang holds that everything in nature has two opposite aspects, namely yin and yang. The opposition of yin and yang is mainly reflected in their ability to struggle with, and thus control each other. For instance, warmth and heat (yang) may dispel cold, while coolness and cold (yin) may lower a high temperature. The yin or yang aspect within any phenomenon will restrict the other through opposition. Under normal conditions in the human body, therefore, a relative physiological balance is maintained through the mutual opposition of yin and yang. If for any reason this mutual opposition results in an excess or deficiency of yin or yang, the relative physiological balance of the body will be destroyed, and disease will arise. Examples are excess or deficiency of yin and yang, or hyperactivity of yang leading to deficiency of yin. "When yin predominates, yang will be diseased; when yang predominates, yin will be diseased."2) The interdependence of yin and yang.Yin and yang oppose each other and yet, at the same time, also have a mutually dependent relationship. Neither can exist in isolation: without yin there can be no yang, without yang no yin. Without upward movement (yang) there can be no downward movement (yin). Without cold (yin) there would be no heat (yang). Both yin and yang are the condition for the other's existence, and this relationship is known as the interdependence of yin and yang. "Yin remains inside to act as a guard for yang, and yang stays outside to act as a servant for yin."3) Yin-yang and the organic structure of the human bodyWhen the theory of yin-yang is applied to explain the organic structure of the human body, the underlying premise is that the human body is an integrated whole. All its organs and tissues are organically connected and can be divided into two opposite aspects, namely yin and yang. In terms of anatomical location, the upper part of the body is yang and the lower part yin; the lateral aspects of the four limbs yang and the medial aspects yin. According to the nature of their functional activities, the zang organs are yin and the fu organs yang. Furthermore, within each of the zang-fu organs, there are tun and yang aspects; for example, heart-yin and heat-yang, kidney-yin and kidney-yang. Within the meridian system there are two categories; yin meridians and tang meridians. Thus the opposition of yin and yang manifests within all the upper, lower, internal and external organic structures. Each contains yin and yang qualities and all of them can be classified according to yin and yang.4) Yin-yang and the physiological functions of the human bodyThe theory of yin-yang holds that the normal vital activities of the human body are based on the coordination of yin and yang in a unity of opposites. Functional activities pertain to yang and nutrient substances to yin. The various functional activities of the body depend on the support of the nutrient substances. Without nutrient substances, there would be no sustenance for functional activity. At the same time, functional activities are the motive power for the production of nutrients substances in the body. In other words, without the functional activities of the zang-fu organs, water and food cannot be transformed into nutrient substance. In this way, yin and yang within the human body are mutually supportive. They act together to protect the organism from invasion by pathogenic factors and to maintain a relative balance within the body. If yin and yang fail to support each other and become separated, the vital activities of the body will cease.5) Yin-yang and pathological changes in the human bodyThe theory of yin-yang is also applied to explain pathological changes. Traditional Chinese medicine considers that the occurrence of disease results from the loss of relative balance between yin and yang, and hence an excess or deficiency of either. The occurrence and development of disease is related both to the antipathogenic qi and to pathogenic factors. There are two types of pathogenic factors: yin and yang. Antipathogenic qi involves yin fluid and yang qi. When yang pathogenic factors cause disease, this may lead to an excess of yang which consumes yin and gives rise to heat syndromes. When yin pathogenic factors cause disease, this may lead to a preponderance of yin which damages yang and gives rise to cold syndromes. When deficiency of yang fails to control yin, deficiency and cold syndromes may appear, in which yang is deficient and yin excessive. When deficiency of yin fluid fails to restrict yang, deficiency and hot syndromes may appear, in which yin is deficient and yang hyperactive. In addition, deficiency of either yang qi or yin fluid may lead to the consumption of the other, known as "mutual consumption of yin and yang." For example, prolonged poor appetite is mainly attributed to weakness of spleen-qi (yang), leading to insufficiency of blood (yin). This is known as "deficiency of both qi and blood due to weakness of yang affecting yin." Another example is hemorrhage, where considerable loss of yin blood usually leads to the syndrome of deficiency of yang, manifesting as chilliness and cold limbs. This is known as "deficiency of both yin and yang resulting from deficiency of yin affecting yang." These pathological changes are all commonly seen in the clinic. (See Figure below for yin-yang balance.)The Five Elements

In early times, the Chinese people recognized that wood, fire, earth, metal and water were indispensible in their daily lives as well as having different natures. For instance, the character of wood is to grow and flourish, the character of fire to to be hot and flare up, the character of earth is to give birth to all things, the character of metal is to descend and be clear, and the character of water is to be cold and to flow downwards. Early doctors applied the theory of the five elements in their extensive study of the physiology and pathology of the zang-fu organs and tissues of the human body, and indeed all phenomena in the natural world that were related to human life. Using analogies, they classified all these, according to their nature, function and form into the five elements. They applied this theory to explain the complicated physiological and pathological relationships between the zang-fu organs, and between the human body and the external environment. The classification of the meridians according to the five elements is based on the nature of the zang-fu organs: