Phlegm is a pathological substance caused by disturbance of body fluid. Generally speaking, the thick part is called phlegm while the thin part is called rheum. Phlegm is either substantial or insubstantial. Substantial phlegm is visible, palpable and audible, such as sputum, scrofula and nodules in the skin and muscles and Borborygmus. Insubstantial phlegm is invisible, unpalpable and inaudible. But there are pathological manifestations of phlegm. Substantial phlegm is identical with insubstantial phlegm in nature. When accumulating, it is substantial; when dispersing, it is insubstantial.

Jingui Yaolue (Synopsis of Golden Chamber) classifies rheum into four categories according to their location, namely phlegmatic rheum in the abdomen, suspending rheum in the chest and diaphragm, sustaining rheum in the rib-sides and overflowing rheum in the skin and muscles.

Phlegm, rheum, water and dampness are all pathological substances caused by disturbance of body fluid. But they are different from each other. The one that spreads and appears substantial is dampness; the one that accumulates and appears substantial is rheum; the one that is thin and clear is water; the rheum that is condensed is phlegm. That is to say that all of them result from disturbance of body fluid and can transform into each other.