Difficulty 36

Each of the five viscera is singular, yet the kidneys alone are two. Why is this?

The answer: The kidneys are two, but they are not both kidneys. The one on the left is the kidney; the one on the right is the life-gate. The life-gate is the place where all spiritual essence and vital qi are stored, and it is also the hinge that sustains the fundamental qi of the human body. Therefore, in men it is used to store essence, and in women it is used to sustain the uterus. Hence it is known that the kidneys are two.