Yin Yang
The theory is based on two basic components: yin and yang, which are neither materials nor energy. They combine in a complementary manner and form a method for explaining relationships between objects. Gradually, this logic was developed into a system of thought that was applied to other areas. Traditional Chinese Medicine (TCM) is an example of one area where the yin yang theory is used to understand complicated relationships in the body.
The original concept of yin and yang came from the observation of nature and the environment. "Yin" originally referred to the shady side of a slope while "yang" referred to the sunny side. Later, this thinking was used in understanding other things, which occurred in pairs and had complementary and opposing characteristics in nature. Some examples include: sky and earth, day and night, water and fire, active and passive, male and female and so on. Working with these ideas, ancient people recognized nearly all things could have yin and yang properties. Yin and yang can describe two relative aspects of the same phenomena such as the example of the slope, or they can describe two different objects like sky and earth.
Usually, yang is associated with functional aspect of an object and has more energetic qualities, for example, moving, ascending, expanding, heat, bright, progressing, active and hyper-functioning states. Yin, on the other hand, is associated with the physical form of an object and has less energetic qualities such as stillness, descending, contracting, cold, dark, degenerating, latent and under-functioning states.