The ancient Chinese proposed that every living thing is sustained by a balance of two opposing forces of energy, called Yin and Yang. Together, they make up the life essence, or Qi - a type of energy that flows through the body via invisible channels called meridians. Half of certain organs and meridians are governed by Yin and the other half by Yang.
When Yin and Yang are out of balance in the body, this causes a blockage of Qi and a subsequent illness. Yin and Yang imbalances can be caused by stress, pollution, poor diet, emotional upsets or infection. For diagnostic purposes, Yin and Yang are further subdivided into interior and exterior, hot and cold, deficiency and excess.