According to the theory of the five elements, wood, fire, earth, metal and water are five basic substances that constitute the material world. These substances are not only of the relations with generation and restriction but set in a state of constant motion and change. That is why it is known as the theory of the five elements or tile doctrine of five evolutive phases. In light of the theory of five elements, TCM has made a comprehensive comparison and study of all kinds of things and phenomena in nature as well as in the viscera, bowels, tissues, functions and forms, thus expounding the physiology, pathology of the human body and the correlation between man and his natural surroundings. The following is a table showing the classification of things according to the theory of the five elements.
Generation implies production and promotion.
As far as the relationship of generation of each of the five elements is concerned, it is composed of two aspects - generating and being generated. The element that generates is called the mother, while the element that is generated is called the son. Hence, the relation of generating and being generated among the five elements is also known as that of mother and son. Take wood for example. Because wood produces fire, it is called the mother of fire.