Yin and yang are the principles for categorizing diseases and also the leading ones in the eight principles. Syndrome differentiation of yin and yang are used in two aspects: differentiating yin syndrome and yang syndrome; differentiating yin asthenia and yang asthenia as well as yin depletion and yang depletion.

Yin syndrome and yang syndrome  
Syndrome differentiation of yin and yang, based on the application of the conception that all things can be divided into two aspects known as yin and yang, generalizes diseases into two categories, i.e. yin syndromes are of yang category; while internal, cold and asthenic syndromes are of yin category. Therefore, yin and yang are the leading ones in the eight principles and include the other six ones.

Yin asthenia syndrome  
Yin asthenia syndrome refers to asthenic heat symptoms due to failure of yin to control yang resulting from deficiency of yin fluid.

Clinical manifestations:
Emaciation, dry mouth and throat, dizziness, palpitation, insomnia, scanty tongue fur, thin pulse, or even feverish sensation over the five centers (palms, soles and chest), tidal fever, flushed cheeks, night sweating, deep reddish tongue with scanty fur and thin and fast pulse.

Analysis of symptoms:
Emaciation, dry mouth and throat, dizziness, palpitation, insomnia, scanty tongue fur and thin pulse are due to malnutrition of the body, viscera and tissues; feverish sensation over the five centers, tidal fever, flushed cheeks, night sweating, reddish tongue with scanty fur as well as thin and fast pulse are due to interior generation of asthenic heat resulting from failure of asthenic yin to control yang.

Yang asthenia syndrome  
Yang asthenia syndrome refers to asthenic cold symptoms due to failure of insufficient yangqi to control yin.

Clinical manifestations:
Pale complexion, dispiritedness, fatigue, shortness of breath, no desire to speak, aversion to cold with cold limbs, spontaneous sweating, moist mouth without thirst, or thirst with preference for hot drinks, clear and profuse urine, loose stool, or scanty? urine with edema, pale, bulgy and tender tongue, whitish slippery fur as well as slow, sunken and weak pulse, etc.

Analysis of symptoms:
Failure of insufficiency of? yangqi to propel and nourish leads to hypofunction of viscera and tissues, giving rise to such symptoms like pale complexion, dispiritedness, fatigue, shortness of breath and no desire to speak; deficiency of yangqi weakens the defensive qi, leading to spontaneous sweating; failure of deficient yangqi to control yin results in internal exuberance of yin cold, bringing about aversion to cold, cold limbs, bland taste in the mouth without thirst or thirst with preference for hot drinks; failure of asthenic yang to astringe and warm causes clear profuse urine and loose stool; scanty urine, swelling and distension are usually caused by failure of asthenic spleen and kidney yang to? warm and transport which leads to internal retention of fluid and edema; pale, bulgy and tender tongue with whitish and slippery fur as well as sunken, slow and weak pulse are signs of internal exuberance of yin cold due to yang asthenia.