Yin-Yang Theory

Yin and Yang are opposites cyclical stages. They form a unity and are complimentary.  Yin contains a seed of yang so that Yin can transform into Yang and vice versa. Nothing is completely yin or completely yang. 

According to TCM there are two forces in the universe, Yin and Yang.  Yin corresponds to cool, water, dark and femininity.  Yang corresponds to warm, fire, masculinity.