In Chinese philosophy, the concept of yin-yang (simplified Chinese: 阴阳; traditional Chinese: 陰陽; pinyin: yīnyáng), which is often called “yin and yang”,1234 is used to describe how seemingly opposite or contrary forces are interconnected and interdependent in the natural world; and, how they give rise to each other as they interrelate to one another. Many natural dualities (such as light and dark, high and low, hot and cold, fire and water, life and death, and so on) are thought of as physical manifestations of the yin-yang concept. The concept lies at the origins of many branches of classical Chinese science and philosophy, as well as being a primary guideline of traditional Chinese medicine,5 and a central principle of different forms of Chinese martial arts and exercise, such as baguazhang, taijiquan (t’ai chi), and qigong (Chi Kung) and of course I Ching.
Yin and yang can be thought of as complementary (instead of opposing) forces interacting to form a dynamic system in which the whole is greater than the parts. Everything has both yin and yang aspects, (for instance shadow cannot exist without light). Either of the two major aspects may manifest more strongly in a particular object, depending on the criterion of the observation.
In Taoist metaphysics, good-bad distinctions and other dichotomous moral judgments are perceptual, not real; so, yin-yang is an indivisible whole. In the ethics of Confucianism on the other hand, most notably in the philosophy of Dong Zhongshu, (c. 2nd century BC) a moral dimension is attached to the yin-yang idea.6
The concept of yin and yang is often symbolized by various forms of the Taijitu symbol, for which it is probably best known in Western cultures. (wikipedia)