In Asian philosophy, the concept of yin yang (simplified Chinese: 阴阳; traditional Chinese: 陰陽; pinyin: yīnyáng), which is often referred to in the West as “yin and yang”, is used to describe how polar opposites 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). Ancient fractal set, Df=1.1.
Yin yang are not opposing forces (dualities), but complementary opposites that interact within a greater whole, as part of a dynamic system. Everything has both yin and yang aspects as light cannot exist without darkness and vice-versa, 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
In Taoist philosophy, shade and light (☯) yin and yang, arrives in the dàodéjīng (道德經) at Chapter 42. It becomes sensible from an initial quiescence or emptiness (wuji, sometimes symbolized by an empty circle), and continues moving until quiescence is reached again. For instance, dropping a stone in a calm pool of water will simultaneously raise waves and lower troughs between them, and this alternation of high and low points in the water will radiate outward until the movement dissipates and the pool is calm once more. Yin and yang thus are always opposite and equal qualities. Further, whenever one quality reaches its peak, it will naturally begin to transform into the opposite quality: for example, grain that reaches its full height in summer (fully yang) will produce seeds and die back in winter (fully yin) in an endless cycle.
It is impossible to talk about yin or yang without some reference to the opposite, since yin and yang are bound together as parts of a mutual whole (i.e. you cannot have the back of a hand without the front). A way to illustrate this idea is to postulate the notion of a race with only men or only women; this race would disappear in a single generation. Yet, men and women together create new generations that allow the race they mutually create (and mutually come from) to survive. The interaction of the two gives birth to things. Yin and yang transform each other: like an undertow in the ocean, every advance is complemented by a retreat, and every rise transforms into a fall. Thus, a seed will sprout from the earth and grow upwards towards the sky – an intrinsically yang movement. Then, when it reaches its full potential height, it will fall.