Difficulty 64

Ten Variations further states: The well points of the yin channels belong to wood, and the well points of the yang channels belong to metal; the spring points of the yin channels belong to fire, and the spring points of the yang channels belong to water; the stream points of the yin channels belong to earth, and the stream points of the yang channels belong to wood; the river points of the yin channels belong to metal, and the river points of the yang channels belong to fire; the sea points of the yin channels belong to water, and the sea points of the yang channels belong to earth. The five-phase attributes of the yin and yang channels are all different. What is the principle behind this?

The answer says: This concerns the matter of hardness and softness mutually complementing each other. The well point of the yin channel is yi wood, and the well point of the yang channel is geng metal. The well point of the yang channel is geng; geng is the hardness of yi. The well point of the yin channel is yi; yi is the softness of geng. Yi belongs to wood, therefore it is called yin well wood. Geng belongs to metal, therefore it is called yang well metal. The remaining points all follow this principle.