Eight principles include exterior and interior, cold and heat, deficiency and excess, yin and yang. Syndrome differentiation of eight principles is to summarize different features of a disease from the eight respects, four pairs of opposite aspects. The exterior and interior are applied to distinguish disease sites and trend of disease development, the cold and heat, deficiency and excess to distinguish disease natures, and yin and yang to summarize, thus to recognize different natures and characteristics of diseases.

Exterior indicates a disease is in the superficial part and the condition is mild; interior means a disease is in the viscera and the condition is severe. Cold and heat are manifestations of superiority or inferiority of either yin or yang. Superior yang gives rise to heat and superior yin gives rise to cold. Deficiency means asthenia of the healthy qi and weakness of resistance; while excess indicates exuberance of pathogenic factors and a fierce struggle between the healthy qi and pathogenic factors.

Generally speaking, a case of acute attack, short duration and superficial site belongs to yang; and a case of chronic attack, long duration and deep site belongs to yin. Viewing a local disorder, an abscess with no ulceration, but with a firm root, high swelling, redness and heat, belongs to excess, heat and yang syndrome; while an abscess with no obvious root, no redness and no heat or little heat, belongs to deficiency, cold and yin syndrome. Clinically, there is often a condition of more than one syndrome occurring at the same time; or transformation of syndrome or false signs.

For example, a pyogenic infection of superficial soft tissue, because of deeper invasion of pathogen, may develop an acute bone infection with a yang syndrome of exterior, excess and heat. As the disease goes on, it may also become a chronic bone infection with a yin syndrome of interior, deficiency and cold. In the severe trauma with great loss of blood, there may occur a syndrome with symptoms of cold limbs and weak pulse hard to be detected, or yin depletion can lead to yang depletion.