According to Traditional Chinese Medical theory, jing or essence can be summarised in two parts: the Yin, being congenital or prenatal, and the Yang, being postnatal or acquired.
The Yang jing circulates through the eight extraordinary vessels and transforms to become and replenish yin; in turn the marrow becomes blood, body fluid and semen.
[2] Jing should not be confused with the related concept of jin (勁; "strength" or "energy"), as in Neijin meaning "internal power", nor with jīng (經; "classic"), which appears in many early Chinese book titles, such as the Huangdi Neijing, Yijing, and Chajing.
[3] The characteristics which constitute signs of good jing (e.g. facial structure, teeth, hair, vitality of adrenals or Kidneys) share the embryological origin of neural crest cells.
[5] An early mention of the term in this sense is in a 4th-century BCE chapter called Neiye "Inner Training" of a larger text compiled during the Han dynasty, the Guanzi.