Yin and yang derives from Daoism which regards them as“the fundamental principles”or”forces in the universe.”In Chinese theory，everything contains these two contracting principles.
Heaven is considered as yang；the earth is yin.Yin and yang are not simply oppose to each other；they coexist in interdependence.Thus，there would be no earth without heaven.Moreover，yin may transform into yang and vice versa.In the humanbody，the upper part is yang，the lower part yin；the exterior is yang，the interio yin；the back is yang，the abdomen yin.With regard to internal organs，the heart，liver，spleen，lung and kidney are yin，while the six intestinal organs are yang.These are the gallbladder，the stomach，the large intestine，the small intestine，the urinary bladder and the triple warmer.
The harmony of yin and yang produces good health；disharmony leads to disease. Therefore，traditional Chinese doctor uses acupuncture，herbs and food to help correct an yin-yang imbalance until the body returns to a healthy state.For example，a preponderance of yang may lead to the hyper-function of an organor heat manifestations. So cooling drugs are used to inhibit the excessiveyang.On the contrary，a preponderance of yin leads to manifestation of cold or weakness in function of an organ.In this case，warming drugs are used to restrict the predominance of yin.
What is the theory of the Five Elements？
The Five Elements are five basic substances of natural phenomenon.These include wood，fire，earth，metal，and water.In accordance with their different properties，functions and forms，traditional Chinese medicine attributes the liver to wood，the heart to fire，the spleen to earth，the lung to metal and the kidney to water.This attribution understands the physiology and pathology of the human body in interrelation with a person and physical nature.The Five Elements are interrelated as follows:Wood begets fire，fire begets earth，earth begets metal，metal begets water，and water，in its turn，nourishes wood.In turn，wood subdues earth，earth subdues water，water subdues fire，fire subdues metal，and metal subdues wood.The same applies to the human physiology:the liver（wood）stores blood to nourish the heart（fire）；the heat of the heart（fire）warms the spleen（earth）；the spleen（earth）transforms and distributes food essence to replenish the lung（metal）；the lung（meta1）dredges the water passages to help the kidney（water）；and the vital essence of the kidney（water）nourishes the liver（wood）.A traditional doctor thus first seeks to nourish the spleen before curing illness in the lung.
