Yin and yang are actually complementary, not opposing, forces, interacting to form a whole greater than either separate part; in effect, a dynamic system. 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. 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 Western world) that yin and yang correspond to evil and good. However, 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 BCE) a moral dimension is attached to the yin-yang idea.