In Taoist tradition, Yin and Yang can be defined as the following:
In Taoism, the main goal is "liberation" of Yang from Yin. In Philosophical Taoism, this is achieved through death. In Religious Taoism, this is attempted by delaying death (that is, by abstaining from sexual desires and other activities believed to contribute to Yin).
The origin of the symbol itself can be traced back to 1000 BCE. Depictions of Yin/Yang have been found on oracle bones (skeletons of animals used in Chinese ritual) dating from this era. It is also believed that the Yin/Yang symbol became widely used in Taoist temples (as they are today), during this time as well. The earliest textual evidence of Yin/Yang can be found in The Book of Songs written by Shijing. It is important to note however, that the origin of Yin/Yang is debatable. Evidence could still resurface, dating this symbol to an older time and era.
Some scholars believe the Yin/Yang symbol itself derives from planetary movements of the sun, moon and four seasons. When charting these positions out on a map, they form the familiar interloping spheres of the Yin/Yang symbol we know today. Ancient Chinese Taoists were very familiar with astrological patterns, and they believed the human body mimicked cosmic activity. Everything has a cycle - the earth, the sun, the moon and the stars. Like planets and stars, human die too. We all return to the one from which we came. It is the cycle of rebirth and death.
In Taoism, the Yin/Yang symbol is called known as the "Taichi." Both spheres contain an element of the opposite: the white containing a small black sphere, and the black containing a small white sphere. One cannot exist without each other, and one contains a part of the other. Life cannot exist without death; darkness without light.
Buddhists believe that when a person dies, their souls are judged according the amount of good deeds they have done. Therefore, the goal is for Yang to triumph over Yin - but in different relation from the Taoist tradition.
Buddhists believe that a person’s good deeds are essential to life. Even if they face misfortune or tragedy, their souls will ultimately be rewarded immortality after death.
This belief essentially answers one of life’s greatest questions: Why do bad things happen to good people? Bad things cannot always be prevented, but those who keep their faith and live accordingly will be rewarded.
The concept of Yin/Yang can be seen in Western Theology, especially that of Mysticism. Mysticism, which incorporates ideas of Chinese Alchemy, also stresses the dual nature of the universe and of man.
Western theory however, denotes these forces as light and dark (Wohl and Wehe). Mystics believe each person has within them both the forces of light and darkness, and ultimately chooses which path he will follow. Light and darkness are co-existing. They cannot exit without each other.
Some Mystic theologians, such as Fludd, believe that "God" contains both these elements - but he is not evil. "God is good, whether he wants or does not want, for in God there is no evil." (R. Fludd, Philosophia Sacra).
Taoism vs. Buddhism and Shinto