 The most common stage involves interaction between two forces. These polar forces could be specified in many ways—heaven and earth, hot and cold, dry and moist—but the pair that came to dominate is yin 陰 and yang陽. (For an authoritative study of yin and yang, see Wang 2012.) Yang originally referred to the south side of a mountain, which received the sun, while yin referred to the north side. Ultimately, yang was associated with the masculine, the forceful, and the bright, while yin was associated with the feminine, the yielding, and the obscure. Creativity followed from the interaction of yin and yang, analogous to sexual reproduction.