Differentiation of syndromes (bian zheng) in TCM is a method to analyse and recognize the syndrome of disease. In other words, it is also a process in which the location, nature, occurrence and development of a disease as well as the condition of vital qi and pathogenic factors are identified according to the clinical data obtained from the four diagnostic methods. From the above it becomes obvious that differentiation of syndromes is the premise and foundation of treatment. Correct differentiation and appropriate treatment are the prerequisite for achieving the hoped-for results. There are a number of methods to differentiate syndromes in TCM, such as differentiation of sytdromes according to the eight principles; differentiation of syndromes according to the theory of qi, blood and body fluid; differentiation of syndromes according to the zang-fu theory; differentiation of syndromes according to the theory of six meridians; differentiation of syndrones according to the theory of wei, qi, ying and xue, and differentiation of syndromes according to the theory of triple energizer. Each method, while having its own features and laying stress on a particular aspect, connects with and supplements the others. They should be applied flexibly and accurately so as to understand a disease comprehensively, thereby providing the basis for treatment.

The eight principal syndrotmes, also known as "eight principles" (ba gang), refer to yin, yang, exterior, interior, cold, heat, deficiency and excess. The differentiation of syndromes according to the eight principles is the general guideline of TDS and important components of TCM diagnostics. Complicated as the clinical manifestations of diseases may be, they are classified under the eight principles. For example, the classification of syndromes - yin or yang: the location of disease-exterior or interior; the nature of disease-cold or heat; and the preponderance or discomfiture of vital qi and pathogens-deficiency of vital qi or excess of pathogen. Differentiation of syndromes according to the eight principles generalizes the intricate and volatile syndromes into four pairs: exterior and interior, cold and heat, deficiency and excess, and yin and yang. Therefore, in the diagnostic process, the eight principles can play a role in bringing out the essentials.

Although this differentiating method classifies various syndromes into eight categories, they are interrelated and inseparable from cach other. For instance, the exterior syndrorme and interior syndrome are related to the cold, heat, deficiency and excess syndromes; the cold syndrome and heat syndrome are related to the exterior, interior deficiency and excess syndromes; the deficiency syndrome and excess syndrome are related to cold, heat, exterior and interior syndromes. These pathologic changes do not occur singly, the exterior and interior, cold and beat, deficiency and excess syndrotnes are usually found simultaneously. For instance, the disease with both the exterior and interior syndromes; the deficiency syndrome mingling with excess syndromes; intertwinement of the cold and heat syndromes. Under certain conditions, these syndromes are often transformed into one another. For example, the exogenous pathogens may invade the interior and vice verse; the cold syndrome rmay be transformed into heat syndrorme and vice versa; the deficiency syndrome may be transformed into

the excess syndrome and vice versa. With the progress of disease, some false appearances contrary to its nature may appear, e. g., cold syndrome with pseudo-heat symptoms and vice versa, deficiency syndrome with pseudo-excess symptons and vice versa.