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Fundamentals of Chinese Medical Theory
At the very foundation of Traditional Chinese Medicine (TCM) lies three general theories:
1. Yin-Yang
2. Five Elements
3. Qi
Yin-Yang Theory
The core concepts of yin and yang reference back to the Zhou dynasty, between 1000-770 BC, where they appear in the ancient Chinese text the Yi Jing (I-Ching), translated as the Book of Changes. A deeply philosophical book of divine nature filled with ancient wisdom and universal truths, the Yi Jing is one of the most influential works ever written in classical Chinese culture (1,2). Between 476-221 BC, the 'School of Yin-Yang' was established. Led by ancient Chinese philosopher Zou Yan, this was one of the 'Hundred Schools of Thought' arising in China at the time (1).
Most people have seen a "yin-yang" symbol in which a circle is divided into two parts with an "S" shape in the middle. On one side it is black with a small white dot; the other side is white with a small black dot. This symbol represents the relationship between yin and yang, especially that of opposition; that yin and yang are opposites of one another.
Yin is dark, inwards, contracting, descending, inferior and below. It represents tangible substances, such as matter, material, structure and form. It is also correlated with all things soft, quiet and slow in nature. As for the human body, its yin aspects are in the frontal and lower region (1).
Yang is light, upwards, expanding, rising, superior and above. It represents immaterial substances, such as energy, air and fire. It is also correlated with all things loud, hard and fast in nature. The yang aspects of the human body are on the back and top regions (1).
All things contain both yin and yang parts in varying amounts, and everything is relatively yin or yang compared to something else. For example, a tree is yang when compared to a flower, but it is yin when compared to a sky-scraper. Also, while the bottom portion of a tree is yin, the top portion is yang. Relative amounts of yin and yang can be broken down into smaller parts for all existing phenomena (1).
Yin and yang could not be without one another, as they rely on each other for their very existence. For example, just as moving, doing, taking action and expending energy (yang) are necessary functions, the body equally depends on adequate rest, relaxation, non-doing and conserving energy (yin) to maintain a state of health (1).
Represented by the black and white dots in the yin-yang symbol, these opposing forces also hold the potential of  transforming into one another. Examples of this are when day turns into night, being in a state of happiness turns into a state of sadness, and so forth. Everything eventually transforms into something else, but only when conditions are right and the potential for change is there (1).
The proportions of yin and yang are in constant flux, aiming to reach a state of greater balance. In this way, they continually adjust their relative levels according to their current environment. When there is a large quantity of one, there becomes a smaller quantity of the other. According to TCM, when these proportions are very out of balance, the disharmony often shows up as the bodily symptoms we see in states of disease. The main goal, therefore, is to bring the levels of yin and yang back into balance (1).
Five Element Theory
The basic theory of the five elements also goes back to the Zhou dynasty, between 1000-770 BC. Like yin-yang theory, the "School of Yin-Yang", led by Zhou Yan, also based its philosophies on this concept. A method of grouping various phenomena into five distinct categories, the five element theory can be applied to many different subjects, including the natural sciences, the calendar, astrology, politics, and of course, the field of medicine. The application of this theory to Chinese medicine marks the beginning of what could be viewed as a shift from mysticism to 'scientific' medicine. Practitioners began debunking supernatural causes of illness and instead started leaning toward natural aspects of health and disease (1).
The five elements are wood, fire, earth, metal and water. Each of these elements functions more as a group of characteristics than as solid states of material substance. They are each comprised of unique qualities, stages, movements and cyclical phases. They naturally exist in specific sequences that can be observed in all existing phenomena. Each of the five elements holds a distinct relationship with the other four, and they constantly exhibit profound influences on one another (1).
When observing the qualities of these elements, Giovanni Maciocia, in his text The Foundations of Chinese Medicine, describes that "wood 'can be bent and straightened', fire 'flares upwards', earth allows for 'sowing, growing and reaping', metal 'can be molded and hardened', and water 'moistens downwards'." Wood is therefore correlated with solidity, fire with combustion, earth with nutrition, metal with moldability and water with fluidity. When viewed as movements, wood expands outwards, fire flares upwards, earth remains stable at the center, metal contracts and water sinks downwards. When viewed as stages of cycles, wood relates to springtime and birth, fire to summer/growth, earth to transformation and what is known as the 'late seasons' or period of days following each of the other four seasons, metal to fall/harvest and water to winter/storage (1).
The five elements form a star-shape when connected together according to their natural sequences.  From a directional standpoint, earth is in the middle, fire is south, water is north, wood is east and metal is west. In this way, all of the elements influence one another by either 'generating', 'controlling', 'over-acting' or 'insulting' the others. The natural order is as follows: wood generates fire, fire generates earth, earth generates metal, metal generates water and water generates wood. In the controlling cycle, wood controls earth, earth controls water, water controls fire, fire controls metal and metal controls wood. The generating and controlling cycles are normal, healthy patterns that aim to keep the system at hand in a state of balance and harmony. In contrast, the over-acting and insulting cycles indicate patterns of pathology in which the system is being overly dominated by one or more elements leading to abnormal relationships between them and subsequent imbalances in the whole. In Chinese medicine, these disturbances, along with yin and yang imbalances, often lead to the symptoms seen in disease (1)
In TCM, each of the five elements correlates with specific organ systems: wood with the liver and gall bladder, fire/heart and small intestine, earth/spleen and stomach, metal/lung and large intestine, and water/kidneys and bladder. According to natural sequence, therefore, the liver and gallbladder generate the heart and small intestine and control the spleen and stomach. When wood is in a state of imbalance, such as in excess, it can negatively impact these neighboring organs, leading to symptoms of disease. These concepts apply to each of the organ systems, which are in a constant state of flux trying to reach greater balance. Chinese medical practitioners aim to regulate organ function in order to establish healthy generating and controlling cycles so the body's entire system can operate at an optimal level (1).
Qi Theory
In Chinese philosophy, Qi is said to be at the foundation of all things in the universe; even yin and yang are simply qi in two unique states, taking form as 'yin qi' and 'yang qi'. It is the incessant assembly and dispersal of qi that makes up all existing phenomena. These phenomena vary greatly according to their degree of materialization or what 'form' they take on. Qi can condense and become material substance or it can disperse and take on non-material forms. This concept can be applied to water (material form of qi) when it evaporates and becomes water vapor in its gaseous state (non-material form of qi). The ancient Chinese also viewed life and death as a form of aggregation and scattering of qi; when condensed it can form a human body, when dispersed one dies and returns to their non-material 'spirit' state (1).
Qi is at the very core of Chinese medicine. TCM acknowledges that although there is ultimately only one qi that makes up all things, there are several different 'types' of it. Each of these 'types' exhibits unique influences on our mind, body and spirit, of which the Chinese believed made up the entirety of a human being. This is due to the fact that qi can take on various roles and functions depending on its specific form, and has the ability to change its form according to the body system in which it resides (1).
In TCM, there are several 'types' of qi that exist within the human mind-body-spirit. Each contains innate characteristics and is in charge of carrying out its own unique functions. For example, two important types of qi are known as 'ying qi' and 'wei qi',  translated as 'nutritive qi' and 'defensive qi', respectively. The nutritive qi is responsible for nourishing the internal organs, flows alongside blood within the vessels and has a close relationship with blood. In contrast, defensive qi is responsible for protecting the body and flows on the outermost parts of the body in the skin and muscle layers. A deficiency or disharmony of any of the different types of qi will lead an individual to display a particular set of symptoms unique to that condition, for which they are treated with a specific protocol (1).
According to Chinese medicine, the qi of the human mind-body-spirit is extracted from three different sources: our parents, the air we breath and the food and drink we consume. This refined qi substance is constantly generated and transformed into the different qi 'types', and all of the vital substances, such as blood and body fluids, that we need to survive. Qi can also perform as a 'function' rather than exist as a material substance. For example, when it resides in a specific organ system, such as the liver, it takes on the normal functions of this organ; that is why we call the collective functions of the liver system 'liver qi'. In TCM, there are twelve major organ systems that are each comprised of their very own 'type' of qi (1).
The ancient Chinese theorized that qi flowed in various 'meridians' or channel systems within the body; along these channels are 'acupuncture points', located on its outermost layers. These points are considered to be small regions where the qi from the internal organs and their respective channel flow to the superficial portion of the body. The use of various devices, such as acupuncture needles, can stimulate these points and manipulate the qi flowing through these channels in order to produce a desired outcome (1).
In summary, the main goals of TCM are to balance the amounts of yin and yang, harmonize all of the organ systems according to five element theory and ensure the adequate generation of qi and all other vital substances in the body. Regular maintenance of these influential properties allows for the smooth flow of qi throughout the human mind-body-spirit, releasing blockages and providing space for healing and transformation to occur (1).
1. Maciocia, Giovanni. The Foundations of Chinese Medicine: a Comprehensive Text. Elsevier, 2015.
2. Lu, D. P. Influence of I-Ching (Yijing, or the Book of Changes) on Chinese Medicine, Philosophy and Science. Acupuncture & Electro-Therapeutics Research, 2013. 38(1), 77-133.
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