In diagnosis:
"The skilled practitioner examines the countenance and feels the pulse. First dividing them into Yin and Yang, he judges the pure (Yang ) and the impure (Yin) and thus knows the diseased part of the body... He feels the pulse to ascertain whether it is floating (Yang), deep (Yin), slippery (Yang) or rough (Yin) and knows where the disease orginated. Thus, no mistake would be made both in diagnosis and in treatment." - Su Wen (Plain Questions)
In the treatment of disease, if Yang is hot and over-abundant, thus injuring the Yin fluid (Yang excess causing a Yin disease), the surplus Yang can be decreased by a method called "cooling what is hot". If Yin is cold and over-abundant, thus injuring the Yang Qi (Yin excess causing Yang disease), the surplus Yin can be decreased by the method called "heating what is cold". Conversely, if Yin fluid is deficient and so, unable to control the Yang, causes it bo become violent; or if Yang Qi is deficient and unable to control the Yang, causes it to become violent; or if Yang Qi is deficient and unable to control Yin, causes it to become over-abundant, then the deficiency must be tonified. The Neijing (Intenral Classic) describes the method thus:---"In Yang diseases treat the Yin; in Yin diseases treat the Yang."
