These two words can possibly be traced back to the Shang and Chou Dynasty,(1550 - 1050BC). But most scholars credit the "Yin and Yang" to the Han Dynasty (206B.C.-A.D. 220). At this time, The Yin Yang School was founded by Tsou Yen. It combines the ancient philosophy of the 5 elements-- wood, fire, earth, metal, water-- and combined them into a cosmology of cyclical movements. The reciprocity between the two poles forms a cycle of movement--or the meaning of change. In relationship to the elements, wood and fire belong to the yang, and water and metal to yin. Earth belongs to both yin and yang because it assists both. The yin and yang only represent opposite poles NOT good or evil.
One of the first religions to use the Yin and Yang symbol was Confucianism. (ca. 179-104BC) The Shih Ching and the I Ching were the first Confucius classics to feature the Yin and Yang. They are represented by the T'ai Chi, a diagram of an egg in which dark and light stand for yolk and white. It symbolizes the origin of all creation. From the egg was hatched the myth of the first man, P'an Ku. This legend dates back to the Shang and Chou dynasty in China. This was a common myth/idea that P'an Ku was born in Chaos resembling a hen's egg. After his birth, eighteen thousand years later, Chaos opened out and the yang, the pure light elements, separated themselves from the yin, the base elements of the earth. P'an Ku grew until he was tall enough to hold heaven and earth apart and his body became the various parts of the earth.
The two are said to be proceeded from the Supreme Ultimate or T'ai Chi, showing the interrelatedness between the two--for as one increases the other decreases. In addition, this symbol shows the perfect balance between opposites, or the great forces of the universe. This portrays that there is no "real" masculine or feminine nature, but that each contains a part of the other. The two are contained in one circle thus showing that both powers are in one cycle. Instead of these two being held in antagonism, they are held together to show the that they are mutually interdependent partners. One cannot exist without the other.
Today, this theory of cosmology has become a philosophy--a way of life--in which the cycle of becoming and dissolution between the world of nature and human events has melded. Many religions and/or philosophies use the Yin-Yang as their symbol--Confucianism, I Ching, Taoism.
The Tao teaches that "all life embodies yin and embraces yang." (Tao Te Ching, Chapter 42) All life, all existence, forms from these two. This can be called Philosophical Taoism. The yin and yang explain the rhythm of the ebb and flow in nature and man. In addition, it expresses the relationship between being and nonbeing. Nonbeing is consider a higher plane than being because it is the source, or the foundation. And from that point of view Yin is valued more than Yang.
In Religious Taoism the concept of the Yin and Yang is again closely related to the 5 elements and how they correspond to the different forces and centers in the human body. Religious Taoism considers the human body a miniature cosmos unto its own, following the cycles of the yin yang movement.
Now, in the Twenty-first Century the philosophy of T'ai Chi has grown to new proportions. In America, we can equate this philosophy to having a successful life during each day ("early to bed, early to rise") Today, the yin-yang symbolizes the dynamic balance of our lives in the world. In this we are to balance and harmonize, not only with ourselves and others, but also with the universe--balancing the active yin with the contemplative yang" (Drehler, 1990). As we begin to find balance in our lives, we become more whole, more complete, and more at peace with ourselves.
To summarize, the Tao Te Ching, Chapter 42 states:
The Tao is the One,
From the One come yin and yang;
From these two, creative energy;
From energy, ten thousand things;
The forms of all creation.
All life embodies the yin
And embraces yang,
Through their union