This method was first put forward by Ye Tianshi (1667-1746 AD), a Qing Dynasty physician, in his book Wenrelun (On Febrile Illnesses). He classified the clinical manifestations of acute febrile diseases(wenbing) caused by exogenous pathogens into four stages: defense(wei), vital energy(qi), nutrient (ying) and blood(xue). These are the four body strata used to explain the location and severity of pathological changes, and form the basis of clinical treatment. This method not only characterizes the four types of febrile disease syndromes, but it is also a reflection of the four grades of severity of febrile diseases during their development.

The defense(wei) stage refers to disorders in the outer defensive forces of the body - protective qi. It usually occurs at the onset of an epidemic febrile disease, the pathological changes are present in the lungs, skin and hair.

The vital energy(qi) stage refers to disorders in the body's inner defensive forces, which manifests as an interior heat syndrome caused by excessive heat evils. In this case, both the body's disease preventing forces and pathogens are very strong, resulting in intense struggling.

The nutrient(ying) stage occurs when pathogenic heat is transmitted into the qi of the blood. Ying is a component of the blood, so the disease is actually located in the heart and pericardium.

The blood (xue) stage represents further invasion of the ying system by the evils. It results in excessive heat stirring the blood and further disturbing the mind. The pathological changes are mainly in the liver and kidneys.