One of several different systems of diagnosis in Chinese medicine. This system can be contrasted with Six Channel Diagnosis, Four Stage Diagnosis, San Jiao Diagnosis, and Zang Fu Diagnosis. The Eight Principles form the foundation of all systems of diagnosis in TCM. The principles are cold vs. heat, internal vs. external, excess vs. deficiency, and yin vs. yang. These four pairs of the Eight Principles are defined in more detail individually.