The etiological factors that cause the occurrence of disease are varied, and these factors may lead to diseases under certain conditions. The ancient Chinese doctors classified the etiological factors in different ways in order to explain their properties and manners for causing diseases. For example, in The Yellow Emperor's Internal Classic, all the etiological factors were classified into two categories for the first time, namely yin and yang. Chapter 62 in Plain Questions points out: "The pathogenic evils either originate in yin or originate in yang; those coming from yang are related to wind, rain, cold, and heat; those coming from yin are related to food and drink, living places, sexual life, and emotions such as joy and anger. ¡°In his book entitled Synopsis of Prescriptions of the Golden Chamber, Dr. Zhang Zhongjing of the Han Dynasty said that all diseases were caused by the evils in three ways: "All diseases and medical problems are exactly within three categories: (1)the meridians are first attacked by the evils, then the viscera are affected; and there is an endogenous cause for this; (2)the vessels and blood circulation that maintain and connect the four limbs and the nine body orifices are blocked, and this is caused by the evils attacking the skin externally; and (3)there are medical problems related to the factors such as sexual activities, injuries by sharp metal things, bites by insect or beast. From this point of view, it can be said that all the etiological causes are included here."

Dr. Tao Hongjing of the Jin Dynasty, in his book entitled A Pocketbook of 100
Recipes, divided all etiological causes into "the endogenous, the exogenous, and others." Dr. Chen Wuze, of the Song Dynasty, extended Dr. Zhang Zhongjing's idea of that "all diseases and medical problems are exactly within three categories," and put forward his "three-factors theory," saying: "The six climatic evils are related to the natural climatic changes; when they overact, they first get into the meridians and collaterals, then into the viscera, so they are exogenous etiological factors. The seven emotions are natural feelings of the human beings; when they arise up, they first affect the viscera, then the exterior of the body, so they are endogenous etiological factors. The others, like hunger, overeating, impairment of qi by yelling, traumatic injuries by metal things, incomplete fracture, infectious pathogens, evil spirit, fright, crushing, drowning, etc., are all beyond the natural scope, so they are neither endogenous nor exogenous factors. "It can be seen that the method used by ancient Chinese physicians to classify etiological factors according to their ways of causing disease possesses a guiding significance for clinical syndrome differentiation.

According to TCM, there is no symptom without any cause, and any kind of symptoms certainly is a reflection of the body being affected and influenced by some pathogenic factors. To understand the causes of diseases, TCM examines the clinical manifestations of diseases and studies the possible pathogenic factors. The procedure of finding the cause of disease through an analysis of the symptoms and clinical signs is termed "finding the cause of disease by syndrome differentiation. The TCM etiology, therefore, studies not 0nly the properties and pathogenic characteristics of etiological factors, but also the clinical manifestations of the syndromes and diseases caused by various etiological factors, in order to direct clinical diagnosis and treatment.