Both wood and fire are above the ground level, which is considered yang. The earth is the ground level, which is neutral. Below the earth are the metal and mineral deposits, which would be regarded as yin. Also, the metal is yin because it is relatively stable compared with the combustible wood. But the most yin of all the five elements is water, which can reach far below the ground level of the earth, lower than the metal.
Water is indispensable in the growth of trees so the cycle repeats with wood.
The historical records of the first century B. C. mentioned Zhou Yen who observed the interactions of yin and yang opposites in nature, the phenomena of the increase and decrease of the yin and the yang, the separation of the Heaven and the Earth, and the movements of the Five Great virtues. He was credited to have perceived the anatomical relationship in terms of a Five-Element System. This is the only historical evidence available. Such a combined yin and yang, Five-Element System constitutes the most important concept in the Cannon of Internal Medicine compiled during the Warring Periods of third century B. C. in China. In fact, such a concept became specialized in the Cannon of Internal Medicine.