The original meaning of the concept of yin and yang is very simple. Initially, the terms referred to whether or not a place faced sunlight. The place that faced sunlight or that was filled with sun was called yang, while the place that faced away from the sun was called yin. It is said in Lu's Spring and Autumn, "The larger the room, the more the yin. The higher the platform, the more the yang. " To early people, yin and yang were merely two aspects of certain things or phenomena. Later, the ancients gradually observed that there were objective existences in the natural world, such as heaven and earth, sun and moon, day and night, cold and hot,bright and dim, death and life, and male and female. From these objective existences,the meanings of yin and yang extended into inter-opposite concepts. Gradually, it was realized that all things in the natural world had two opposite aspects, and that the interaction of these two aspects promoted change and development. As a philosophic conception, yin-yang is first mentioned in the Zhou Yi. It says, "The combination of yin and yang is called the law. " It shows that the ancients had gradually realized that the world is material, and that the material world itself is the result of the unity of opposites between yin-qi and yang-qi. During the periods of Spring-Autumn and Warring-State, there was much productive observation of natural phenomena, as well as extensive practice of the yin-yang theory. The motions of the sun, moon, and stars, as well as the alternate climatic changes between hot and cold, make up the movement of materials in the natural world. There are four basic patterns of motion and life activity: ascending, descending, exiting, and entering. Thus, it was further realized that both the movement of materials within the natural world and the life activities of organisms were a combination of two opposite forces of yin and yang. These forces were themselves consisting of inter-rejection,inter-dependence, inter-opposition, and inter-promotion. Hence, the ancient viewpoint of the unity of opposites was formed. Yin and yang were used to represent the relationship of the unity of opposites between two contradictory aspects of phenomena and were used to expound the law of things, movement, and change. Thus, yin and yang are two basic categories abstracted from the complexity of phenomena and they penetrate into all things. This concept has been further abstracted and deepened so as to develop into the theory of yin-yang. The concept of the unity of opposites between yin and yang was absorbed by the ancient physicians and combined with their long-term accumulated knowledge of anatomy,physiology, and the prevention and treatment of disease. The result was TCM's formalized theory of yin-yang which is used to expound the basic contradiction of life phenomena and the objective laws of life activities. But it should also' be pointed out that what the theory of yin-yang in TCM reflects is merely some objective laws about life science, but not the general movement laws of the natural world, human society, and thinking activities. While the concept of yin and yang is all-embracing, it refers only to specific phenomena which is contradictory in nature and exists concretely within the objective world.
