Through long-term observation, analysis and synthesis, it was concluded that a type of "defence-qi" is disseminated along the exterior of the body and performs the role of muscular protection, guarding against ex-ogenous evils.

Generally, it is not easy to be attacked by exogenous evils when awake or moving about. On the contrary, one may catch a cold while sleeping. Thus, it is clear that the circulation of defense-qi is closely related to the awake or sleep state, and therefore defense-qi circulates at night and is the yin aspect, while during the day it is the yang aspect. Another example is that anxiety often produces poor appetite, dyspepsia, abdominal distention, fullness, etc. Long-term observation of this correlation led ancient Chinese to connect "pensiveness", mental stress, to the digestive function of the spleen. Hence, the viewpoints of "The spleen dominates pensiveness", and "Excessive pensiveness injures the spleen", were held.