"Yin and Yang" is the general principle of the eight cardinal principles. It first classifies the data from the previous four diagnoses into two categories: "Yin Syndrome" and "Yang Syndrome", and then goes down step by step.
"Exterior and interior" refers to the "disease position" of a disease. However, it should be noted that "exterior" and "interior" are not only specific "body surface" and "body interior", but also a special term for classification. For example, the fever and cough in the early stage of a cold are diagnosed as "exterior syndrome" by traditional Chinese medicine. However, from the perspective of modern medicine, the "heat" is also from the inside of the human body rather than from the "skin fever". Cough is more from the trachea, throat and other places rather than "skin cough". Therefore, "exterior syndrome" is not equal to "epidermal disease". This phenomenon is common in traditional Chinese medicine, such as "Qi" is not equal to "gas" and "wind" is not equal to the wind blowing from outside. We should be careful to understand it.
"Cold and heat", which explains the nature of the disease, is also a TCM term. "Cold" is not equal to "low temperature", and "heat" is not pure "high temperature".
"Deficiency and excess" is a summary of the state of the body's reaction after getting sick - "deficiency" is "deficiency", which is equivalent to the low function and resistance of the body; on the contrary, "excess" is "excess", which is the state of hyperactivity of the body. We need to understand its essence.
After the "Eight Principles" syndrome differentiation, combined with more detailed syndrome differentiation methods such as "six meridians syndrome differentiation", "Wei Qi Ying blood syndrome differentiation", "Sanjiao syndrome differentiation", "viscera syndrome differentiation", etc., we can make a clear final diagnosis.