The Chinese Taoists, like the ancient Egyptians, were greatly concerned with ensuring the survival of the individual after physical death. Their metaphysics was based on the ancient Chinese conception of the polarity of Dark and Light, Negative and Positive, Yin and Yang; the two fundamental principles in the Cosmos.
According to the Chinese, just as the Cosmos consists of and comes about through the interaction and interchange of Yin and Yang (as superbly illustrated in that magnificent Chinese Oracle, the I Ching - pronounced "Yee Jing"), so, in a similar way, the human personality consists of and comes about through two principles or "souls", a Yin soul and a Yang soul, which are welded together during life, but separate at death. Their separation means the end of the personality as such, even though the Yin and Yang principles survive.
Yin and yang are the most familiar terms to the Western public, being equally, or even better known than Tao-te ching or Lao-tzu. In Eastern thought, the two complementary forces, or principles, that make up all aspects and phenomena of life. Yin is conceived of as earth, female, dark, passive, and absorbing; it is present in even numbers, in valleys and streams, and is represented by the tiger, the colour orange, and a broken line. Yang is conceived of as heaven, male, light, active, and penetrating.
As a matter of fact, yin and yang are not mysterious, at least not for the Chinese. It is the multitude of meanings attributed to them that stirred confusion, attracting a sense of eccentricity.
On the other hand, this couple of opposite terms sounds quite familiar to the modern mind, which is intoxicated with dialectic. Dualism surpassed its religious, heretic stage and is now located in the realm of modern psychology and philosophy. We are only flattered to find this couple of opposites in the vocabulary of the Chinese philosophy: it proves that wherever we look, there is a single piece of truth - our truth.
So, what we have is the mysterious on one side, and the familiar, on the other. The success of this pair of terms is understandable now.
According to Legge, yin and yang are not such old as one may think. They are not referred to directly in I-ching, except for the commentaries (Shih-I) on it.
Legge also elucidates the meaning of yin and yang, derived from the sun and the moon, translated as bright and dark (1).
In Book 5, Chapter 2 from Lu-sih ch'un-ch'iu (Spring and Autumn Annals), we find a description of the way all things were generated from yin and yang:
The Great One produces the two poles [i.e. Heaven and Earth], which in turn give rise to the energies of the dark (yin) and the light (yang). These two energies then transform themselves, one rising upwards, and the other descending downwards; they merge again and give rise to form. (2)
Lao-tzu mentions yin-yang polarity only once, in chapter 42 of Tao-te ching: