The Eight Principles are the fundamental concepts of diagnosis in TCM. They form the foundation for all methods of pattern identification and formulation.
Patterns belonging to the Eight Principles should not be viewed as “either-or”. It is not uncommon to see conditions that are both interior and exterior for example. Not all clinical conditions will contain all four characteristics. Blood deficiency serves as a good example as it does not involve Hot or Cold.
Yin and Yang are regarded as the rulers of the Eight Principles. External, Excess and Heat belong to Yang, whereas, Internal, Deficient and Cold belong to Yin.