The Five Phases are called the Five Elements, but the system of five phases does not describe static elements, rather interactions between phenomena. It was employed as a device in many fields of early Chinese thought, including seemingly disparate fields such as geomancy, astrology, music, military strategy, martial arts, and traditional Chinese medicine.
The theory of five phases describes two cycles of balance, a creation cycle, and an overcoming cycle. These are illustrated in a chart like the one presented here:
According to Wu Xing theory, the structure of the cosmos mirrors the Five Phases. Each phase has a complex series of associations with different aspects of nature. In ancient Chinese geomancy (Feng Shui) these phases are associated with the trigrams of the Bagua. In Chinese medicine the body is considered a circle of five things, and so mapped by the Chinese doctors onto the five phases. For instance, the Liver (Wood phase) is said to be the mother of the heart (Fire phase), and the Kidneys (Water phase) the mother of the Liver. The Five Phases model of the human body, like all models, has exceptions, but—generaly—the model seems to be useful for arriving at good clinical results, so they were kept by the critically thinking Chinese medical doctors and researchers since they were first introduced with The Yellow Emperor's Classic of Medicine.