Silence lingered in the valley for a long time before Qian Boya finally spoke first and broke it. "May I ask you, Grand Master? An alchemical scripture says, 'Zi in the south, Wu in the north, each governing the other.' Yet Zi belongs to the night, when the moon rises, and is yin in nature, so it should correspond to water. Wu belongs to the day, when the sun is at its height, and is yang in nature, so it should correspond to fire. By your explanation, Zi should be in the north and Wu in the south. Why does the scripture describe them in reverse?"
Cheng Xinzhan smiled at this. He had not expected City Lord Qian to have studied the Dao of Alchemy, though apparently not very deeply. He answered, "That comes from The Seal of the Unity of the Three. At the Zi hour, a spark of yang is born; flourishing yin begins to decline, and yang arises at yin's extreme. This is 'the south concealed within the north.' At the Wu hour, a spark of yin is born; flourishing yang turns toward yin, and yin arises at yang's extreme. This is 'the north concealed within the south.' The passage means taking from Kan to fill Li: gathering the yang of northern Kan to kindle the yin of southern Li. Heh, alchemists tend to be interesting people, fond of fine writing and metaphors. There is nothing strange about their reversing the mysteries."
Qian Boya nodded slowly to acknowledge the lesson, then asked again, "I have seen an alchemical formula that says, 'The east's three and the south's two together make five; the north's one joins the west's four.' How should its numerical principles and the directions of the five elements be understood?"
Cheng Xinzhan answered offhandedly, "That