The Duad

Duad is in reference to the couple; two items of the same kind; often thought of as yin-yang, the couple or two items of the same kind or that which is nature. The way of nature is the interplay of the light and dark as represented in the symbol of yin-yang. The two principles that combine in various stages or modulations of light and dark which is that we call the world and its way, the principles of the yin and the yang.

In ancient Chinese thought the yang and yin in their origins refer to the sunny and shady side of the stream; the yang being the sunny side, and the yin being the shady side.

As stated in the ken-po goku-i as well as other more in depth ancient Chinese classics, Earth and Sun are associated respectively with the feminine and the masculine "principles" and with the passive and the active principles.

Light is not better than dark nor dark is not better than light; just two principles on which the world rests: the light and the dark. The great Tai Chi or the great Duad, yang and yin.

Bibliography:
Campbell, Joseph. "Myths of Light: Eastern Metaphors of the Eternal." New World Library. Novato, California. 2003.