《黄帝内经·素问·方盛衰论》
黄帝回答道：阳气主升，其气从左而右；阴气主降，其气从右而左老年之气先衰于下；少年之气先盛于下，其气从下而上。
黄帝答曰：阳从左，阴从右，老从上，少从下。
The Yellow Emperor replied: Yang qi tends to rise, its qi goes from left to right; yin qi tends to descend, its qi goes from right to left. The qi of old people first decays below; the qi of young people first flourishes below, its qi goes from below to above.