Chapter four of the Nei Jing, Su Wen, entitled, “金贵真言论” or “True Sayings form the Golden Cabinet” provides the following philosophical descriptions that will give the basis to understanding disease progression.
They start from the very beginning, Yin and Yang; “阴阳者， 天地之道也，万物之纲纪，变化之父母， 生杀之本始， 神明之府也，治病必求于本。。。阳化气， 阴成形。寒极生热， 热极生寒。” “Yin and Yang are the way of heaven and earth, the primary discipline of the myriad things, the mother and father of change, the root of birth and death, the god’s palace also. To cure disease one must seek out its root… Yang transforms qi, Yin becomes form. Extreme cold creates heat, extreme heat creates cold.”
This passage presents the presuppositional view that dualistic Yin and Yang are the ultimate laws of the universe. Here heat is represented by Yang and cold is represented by Yin. Yet, an overabundance of Yin can cause a transformation of Yin into Yang. They can transform into each other, like the Tai Ji diagram suggests. They have a dynamic relationship where change is the only constant.
Fig. 太极，or Tai Ji diagram. Commonly referred to as Yin and Yang. Showing the interconnectedness between these two aspects of the universe.
Nei Jing Eastern Medicine 