Yin and Yang on black flag
In Chinese philosophy, the concept of yin yang, normally referred to in the West as "yin and yang" is used to describe how polar or seemingly contrary forces are interconnected and interdependent in the natural world, and how they give rise to each other in turn. Opposites thus only exist in relation to each other. 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, and a central principle of different forms of Chinese martial arts and exercise, such as baguazhang, taijiquan (tai chi), and qigong (Chi Kung) and of I Ching divination. Many natural dualities—e.g. dark and light, female and male, low and high, cold and hot— are thought of as manifestations of yin and yang (respectively).
Yin yang are complementary opposites that interact within a greater whole, as part of a dynamic system. Everything has both yin and yang aspects, but either of these aspects may manifest more strongly in particular objects, and may ebb or flow over time. 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.
There is a perception (especially in the West) that yin and yang correspond to evil and good. However, Taoist philosophy generally discounts good/bad distinctions and other dichotomous moral judgments, in preference to the idea of balance. Confucianism (most notably the philosophy of Dong Zhongshu, c. the 2nd century BCE) did attach a moral dimension to the idea of yin and yang, but the modern sense of the term largely stems from Buddhist adaptations of Taoist philosophy.