One of several different systems of diagnosis in Chinese medicine. This system can be contrasted with Eight Principles, Six Channel diagnosis, San Jiao diagnosis, and Zang Fu diagnosis. Four Level diagnosis deals with the invasion of the body by warm-natured External Pathogenic Factors (see above). It outlines how a pathogen invades the body from the exterior levels to the deep internal levels, level-by-level; how to diagnose and treat the disease at each level, and how the natural untreated progression of the disease will eventually lead to death.