"The Five Elemental Energies of Wood, Fire, Earth, Metal, and Water encompass all the myriad phenomena of nature.  It is a paradigm that applies equally to humans."

The Yellow Emperor's Classic of Internal Medicine

The Five Elemental Energies (Wu Xing) represent the tangible activities of yin and yang as manifested in the cyclic changes of nature which regulate life on earth.  Also known as the Five Movements (wu yun), they define the various stages of transformation in the recurring natural cycles of seasonal change, growth and decay, shifting climatic conditions, sounds, flavors, emotions, and human physiology.  Each energy is associated with the natural element which most closely resembles its function and character, and from these elements they take their names.  Unlike the Western and other systems of five elements, the Chinese system focuses on energy and its transformations, not on form and substance.  The elements thus symbolize the activities of the energies with which they are associated.

 As manifestations of yin and yang on earth, the Five Elemental Energies represent various degrees of 'fullness' and 'emptiness' in the relative balance of yin and yang within any particular energy system. 

An ancient Chinese text explains this principle as follows:

By the transformation of yang and its union with yin, the Five Elemental Energies of Wood, Fire, Earth, Metal, and Water arise, each within its specific nature according to its share of yin and yang.  These Five Elemental Energies constantly change their sphere of activity, nurturing and counteracting one another so that there is a constancy in the transformation from emptiness to abundance and abundance to emptiness, like a ring without beginning or end.  The interaction of these primordial forces brings harmonious change and the cycles of nature run their course . . . The Five Elemental Energies combine and recombine in innumerable ways to produce manifest existence.  All things contain all Five Elemental Energies in various proportions.