《黄帝内经·灵枢·岁露论》
少师说：时逢岁气不及的虚年，遇上月亮亏缺无光，四时气候失和，在这种条件下，最容易感受贼风邪气的侵袭，这种情况称为三虚。
少师曰：乘年之衰，逢月之空，失时之和，因为贼风所伤，是谓三虚。
The lesser tutor said: In the years when the annual qi is deficient and the moon is crescent, the four seasons' climate is out of harmony; under these conditions, the human body is most likely to be invaded by evil wind and pathogenic qi. This is called "three vacuities".