The [morally] noble woman guards herself against three things. When she is young, her xue-qi has not yet stabilized, so she guards herself against sexual passion. When she reaches her prime, her xue-qi is not easily subdued, so she guards herself against combativeness. When she reaches old age, her xue-qi is already depleted, so she guards herself against acquisitiveness.

   QI (chi) - The Energy Of Life