The Yinyang concepts "originate in ancient Chinese philosophy and metaphysics, which describes two primal oppsoing but complementary forces found in all things in the universe. Yin, the darker element, is passive, dark, feminine, downward-seeking and corresponds to the night; yang, the brighter element, is active, light, masculine, upward-seeking and corresponds to the day" (Wikipedia.org). YinYang may even go back in time as far as ancient Neolithic Chinese agrarian religions, but in later pre-Christian times was present in Confucianism and in the Tao Te Ching (although the words themselves only appear once in the book, there are references everywhere to the principles). The Yellow Emperor (2698 BC to 2598 BC) was a major exponent of yingyang, and its relationship to Chinese medicine.

During the Qin dynasty in China between 221BC to 207BC, a significant and often ruthless degree of central government was established, along with am attempt to standardise the culture and philosophy of the region. This standardisation was executed by making illegal the schools of thought based upon Confucius, Lao Tze and Mo Tzu in favour of their own Legalist philosophy – a rejection of the varied and rich ways of thinking that had marked out the previous three hundred years.

As the Han dynasty under Emperor Gao (Liu Bang) took over from the fall of the Qin in 207BC, they adopted many of the concepts of central government set up by their predecessors, and agreed with a standardisation of culture – but not with the exclusivity of a single school of thought. Rather, they attempted a fusion of the varied philosophies (including the previously illegal ones), concentrating on the classics such as the I-ching, and distilling a Taoist yin yang school of Chinese thought. This ‘Taoism era’ brought wealth, prosperity, peace and stability to the region, through a revival of agriculture, relief of harsh laws and conditions and threats from others. Later, the emperor Wu moved the Dynasty to a Confucian state.

The meaning of yinyang is inherent in and illustrated by the symbol for it. The circle around the outside represents the Tao or single principle, great void or Great Ultimate; and from this single principle comes everything.

This principle is then divided into two opposite principles that interact with each other: yin (the black half) and yang (the white half). These two opposing principles provide everything – an analogy would be that all the millions of colours found in nature (and with a computer you can choose from a palette of 16.7 million) are produced from the three primary pigment colours of red, yellow and blue (or red, green and blue if you are looking at the optical spectrum). The yin and yang principles accomplish the changes in the universe through the five agents (elements) of wood, earth, fire, water, and metal. The Taoists can explain the workings of the universe through the yin and yang interactions of the five agents as they interchange, oppose, merge, yield to, overcome, birth, feed off one another.

With the yang principle come the more outward aspects: maleness, creation, upwards, dominance, active, hot, expanding, strong, and so on and in tai chi yang is the solid outward pushes, out-breaths, and attacking moves. The yin on the other hand, indicates aspects of femaleness, downward-movement, moon, cold, submission, dark, passive, contracting weak, and so on; and in tai chi yin are the inward defensive movements, in-breaths.

That is not to say that yin is the weaker partner: Cheng Mang-chin’s form of tai chi chuan used the yin principles of yielding to use the opponent’s yang attacks and strength against them to achieve successful outcomes. This is illustrated in the yin-yang symbol: instead of an absolute and static line divided down the middle, the division is instead flowing curves – each side not only pushes into and dominates the other, but also yields and is dominated by the other. There is no antagonism here between the two (such a dark/light concept is more to be found in the apochryphical religions of the West and Middle East) but a complementary existence. Each nourishes, sustains and controls the other, and each depends upon the other to exist. So by illustrating these opposites in this way, they are shown as complementary, rather than absolutes. The Taoist principles of yin/yang states and the way that they describe everything in nature show the two in constant movement. It also shows very effectively that they gradually change into each other – yin eventually changes into yang, yang eventually metamorphoses into yin.

Into these constantly moving opposites comes balance and equilibrium. In the symbol we see a small circle or dot of the opposite colour within each of the halves – again a reinforcement that there are no absolutes in yinyang. Bluntly put, in every yang there is a little yin, and vice-versa. This can be extrapolated out to all aspects of the universe: in every male there is a little female, in every light there is a little dark, and in every evil there is a little good, and so on. In tai chi chuan, a yin movement of grasping an opponents incoming fist (yang punch) and yielding your body to allow your opponent’s own momentum to take the fist (and its attached owner) past out of harms way and into an overbalanced vulnerable state is helped on its way by a little yang pushing…

The Yellow Emperor, legendary Chinese emperor and cultural hero ruling from 2698 BC to 2598 BC and considered to be the ancestor of the Han Chinese, said “The principle of Ying and Yang is the foundation of the entire universe. It underlies everything in creation. It brings about the development of parenthood; it is the root and source of life and death…

“Heaven was created by the concentration of yang, the force of light, earth was created by the concentration of yin, the force of darkness. Yang stands for peace and serenity, yin stands for confusion and turmoil. Yang stands for destruction, yin stands for conservation. Yang brings about disintegration, yin gives shape to things…”

bib:
An essay on the Chinese metaphysical concept of the two opposites, yin and yang, and their origin in the philosophical speculation of the Han Synthesis. Chinese philosophy learning module auth. Richard Hooker. http://www.wsu.edu:8080/~dee/CHPHIL/YINYANG.HTM
The meaning of Yin-Yang from a translation by Mark Coyle, on a personal website of Svebor Hlede http://fly.cc.fer.hr/~shlede/ying/yang.html
Yin and Yang in Medical Theory, from Patricia Ebrey, Chinese Civilization: a Sourcebook, 2d ed. (New York Free Press 1993). also from the Hlede website.
Various extracts from Wikipedia.org.
The Interelationship of Yin-yang http://asiarecipe.com/yinyang.html