External and internal are two principles used to differentiate the location of diseases and the tendency of pathological changes. External and internal are two relative concepts. On the human body, the skin, hair, muscular interstices and shallow meridians and collaterals are external; while viscera, qi, blood and bone marrow are internal.

External and internal differentiation of syndromes is important to syndrome differentiation in exogenous diseases. This is because the disorder due to internal impairment starts from the interior and does not show the course of developing from the external to the internal. In this case there is no need to differentiate the external and internal. In exogenous disease, when the pathogenic factors often invade the human body, they first attack the superficies. In this case, the healthy qi fights against the pathogenic factors, giving rise to the formation of external syndrome. With the development of the pathological conditions, pathogenic factors transmit from the exterior into the interior and from the shallow layer into the deep layer to form internal syndrome. Therefore, external and internal syndrome differentiation is the most basic cognition of the developing stages of exogenous diseases. The external and internal syndrome differentiation enables doctors to understand conditions of pathogenic factors and the states and development of pathological changes so as to take proper and timely treatment.

External syndrome
External syndrome refers to the symptoms appearing at the primary stage of exogenous diseases caused by invasion of six pathogenic factors into the body through skin, mouth and nose, marked by sudden onset, short duration and shallow location.
 
Clinical manifestations:
The clinical manifestations are fever, aversion to cold (or aversion to wind), thin and white fur and floating pulse, accompanied by stuffy and running nose, sore-throat and cough.

Analysis of the symptoms:
Attack of pathogenic factors against the superficies and confliction between healthy qi and pathogenic factors lead to fever and aversion to wind and cold; stagnation of pathogenic factors in meridians prevents meridian qi from free flowing and results in pain. the lung governs skin and hair, the nose opens into the lung and the throat is the door of the lung, so attack of pathogenic factors against the surface of the body leads to dysfunction of the lung and causes stuffy and running nose, sore-throat and cough; the pathogenic factors retain in the superficies and have not damaged the interior, so the tongue fur is still thin and white without change; floating pulse is the sign of external confliction between the healthy qi and pathogenic factors.

Internal syndrome  
Internal syndrome refers to the symptoms in disorders with deep location (such as disorders of viscera, qi and blood and bone marrow), usually seen at the middle and advanced stages of exogenous disease and the whole course of diseases due to internal impairment. Three conditions have contributed to the formation of internal syndrome: further development of exogenous disease due to interior transmission of pathogenic factors from the exterior and invasion of the viscera by the pathogenic factors; direct attack of the viscera by pathogenic factors; dysfunctions of the viscera and the imbalance between qi and blood due to impairment of the viscera caused by emotional upsets, improper diet and improper daily life.
 
Clinical manifestations:
The clinical manifestations of internal syndrome are different due to different causes and location. Since syndrome is either cold or heat and asthenia or sthenia and since disease is due to the disorders of either qi or blood or body fluid, clinical manifestations of internal syndrome are various. However, the basic clinical manifestation is dysfunction of the viscera which will be discussed in the following sections. Here sthenic internal heat syndrome in exogenous disease is taken as an example to illustrate the clinical manifestations of internal syndrome. The basic manifestations are high fever, aversion to heat, restlessness, even coma with delirium, thirst with profuse drinking of water, scanty and brownish urine, retention of feces, reddish tongue with yellowish fur and fast and powerful pulse.
       
Analysis of symptoms:
Exuberance of internal heat leads to high fever and aversion to heat；heat disturbing the heart spirit causes restlessness or even coma with delirium; consumption of body fluid by exuberant heat leads to thirst with profuse drinking of water, scanty brownish urine and retention of feces；reddish tongue with yellow fur and fast powerful pulse is the sign of exuberance of internal heat and confliction between healthy qi and pathogenic factors.

Appendix:
Half external and half internal syndrome Half external and half internal syndrome refers to the symptoms appearing in exogenous disease at the stage marked by confliction of healthy qi and pathogenic factors between the exterior and interior phases, and is usually caused by transmission of pathogenic factors from the exterior to the interior but still lingering between the exterior and interior phases. The manifestations are alternation of cold and fever, oppression and distress over the chest and hypochondrium, dysphoria, susceptibility to belching, silence, anorexia, bitter taste in the mouth, dry throat, dizziness and taut pulse, etc. (see shaoyang syndrome in syndrome differentiation of six meridians).