The taijitu or yin-yang symbol of Chinese philosophy expresses the flowing nature of the interplay of opposing forces of any kind. The simple scheme based on black and white pushes abstraction to its limits. Although yin can be associated with the feminine, with shaded valleys, with earth; and yang with the masculine, with brightly lit places, with heaven; none of these associations reduces yin or yang to anything in particular. And opposing forces can be represented here and the divine union of male and female deity becomes just one more representation of a much more abstract or symbolic affair.
Much of Taoist erotic art is not at all erotic to the untrained Western eye, precisely because of this push toward the abstract and symbolic. There are dragons flying among clouds, there are peaches and mushrooms, flowers and birds. At most, it appears to be decorative, nothing more. And yet there is sexual symbolism everywhere. It is just symbolism of an esoteric kind.
In the I Ching, the Chinese book of divination, we meet a hexagram of a different sort. Here, it is a stack of six lines, some representing yin, some yang, some fixed, some changing. With four possibilities in six places, a total of 64 hexagrams can be produced. The stack is also commonly discussed as two trigrams, one over the other, of which there are eight. For example, hexagram 12 called “Obstruction” or “Standstill” appears like this:
The top three unbroken (yang) lines make up the trigram for “heaven” while the bottom three broken (yin) lines make up the trigram for “earth”. The lower trigram is deemed to be “inner”, the upper to be “outer”. This image is described thus:
Heaven and earth do not unite: The image of Standstill. Thus the superior man falls back upon his inner worth in order to escape the difficulties. He does not permit himself to be honored with revenue.
When the trigrams are reversed, we arrive at hexagram 11 called “Pervading” or “Peace”:
So, clearly it is seen as preferable to have heaven within and earth outside.
These ideas were used to produce two “Taoist” magens, one representing more feminine and heavenly aspects, the other more masculine and earthly.