There exists two opposing and interrelating forces in our Universe. The Yin and Yang are natural forms of energy that are in a constant state of change, movement and interaction. This dynamic relationship creates and governs all life. From Birth, Growth, Sickness and Death, the stages of existence are in constant evolution.
The interaction of Yin and Yang is reflected by Night and Day, Positive and Negative, Active and Passive, Hard and Soft, Fast and Slow, Male and Female, Hot and Cold, Anger and Happiness, and so forth. We can see this interplay in nature in the form of the 4 seasons where yang is reflected as the peak of summer and yin in the coldness of winter.
Yin and Yang are therefore the two primordial forms of Qi. The interaction between Yin and Yang subsequently relates to the Five Phases of Qi. These are later known as the Five Elements.
In Feng Shui, yin and yang takes on a different dimension. The mountain or landscape, which is unmoving and still is classified as Yin. The rivers, sea or lakes which contains active, moving water, is classified as Yang. Thus, a general rule for an ideal site would be a place where the rivers and hills converge harmoniously – a matrimonial combination of yin and yang. Of course, this goes further on as we start to analyze the qualities of the mountain (be it edged, sharp, rocky or undulating) and water (be it rushing, fierce, loud, calm or still).
Chinese sages state that the Tao or Tai Ji comes from the Great Void (Nothingness). The great void is known as the WuJi. The WuJi is the netherworld of all creation and the pure land of all wisdom and life in the Universe.