The nature of yin-yang
Yin-yang is an active concept: yin and yang are thought to arise together from an initial quiescence or emptiness (wuji, sometimes symbolized by an empty circle), and to continue moving in tandem 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-yang, thus, always has the following characteristics:
Yin yang are opposing
Yin yang describe opposing qualities in phenomena. For instance, winter is yin to summer's yang over the course of a year, and femininity is yin to masculinity's yang in human relationships. It is impossible to talk about yin or yang without some reference to the opposite.
Yin yang are rooted together
Since yin and yang are created together in a single movement, they are bound together as parts of a mutual whole. A race with only men or only women would disappear in a single generation, but 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 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 height, it will begin to weaken, and eventually will fall back to the earth in decay - an intrinsically yin movement. Yin always contains the potential for yang, and yang for yin.
Yin-yang are balanced
Yin-yang is a dynamic equilibrium. Because they arise together they are always equal: if one disappears, the other must disappear as well, leaving emptiness. This is rarely immediately apparent, though, because yang elements are clear and obvious while yin elements are hidden and subtle.
Yin-yang is not an actual substance or force, the way it might be conceived of in western terms. Instead, it is a universal way of describing the interactions and interrelations of the natural forces that do occur in the world. It applies as well social constructions - e.g. value judgements like good and evil, rich and poor, honor and dishonor - yet it is often used in those contexts as a warning, since by its principles extreme good will turn to evil, extreme wealth to poverty, extreme honor to dishonor.
The concept of "unity in duality" arises in many faiths and philosophies, from the philosophy of Heraclitus, to the nondualistic philosophies of Hinduism,Sikhism,Taoism,and Buddhism, to Gnosticism and Zoroastrianism. Yin-yang is unique, however, both in its dynamic nature and its broad application to the natural world.