In the FOUR GROUPS OF FUNDERMENTAL ELEMENTS OF SYNDROME DIFFERENTIATION, it is thought that HEALTHY QI and PATHOGENIC QI are the leading principles; MOISTENING, UNIMPEDEDNESS and LIMPIDNESS are the regular and normal manifestations of a healthy human body and the correct ways of treatment; and also, DRYNESS, OBSTRUCTEDNESS and TURBIDNESS are the irregular and abnormal manifestations of a sickened human body and the courses of diseases. HEALTHY QI and PATHOGENIC QI are to differentiate the prevention, the development and the declining of diseases; while MOISTENING and DRYNESS are to identify the flourishing and the waning of human body fluids. UNIMPEDEDNESS and OBSTRUCTEDNESS are to distinguish the smoothing and blocking QI inside human bodies; in the meantime, LIMPIDNESS and TURBIDNESS are to recognize the clearness and the thickness of the QI inside human bodies.