Man has departed from nature and must return now and be in union with it. To be in harmony with the vastness of nature, is to be in communion with the Tao, which ultimately is union with the I Am. The concept of Five Elements is an outcome of yin and yang interactions. The five elements are wood, fire, earth, metal and water. Wood is active, capable of growth and combustion, which is yang.
Fire is most active, requiring air for combustion which is most yang.
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.