《黄帝内经·素问·太阴阳明论》
所以，阳气性质刚实固守于外，阴气性质柔虚守于内。
故阳道实，阴道虚。
Therefore, Yang Qi has the characteristic of hardness and firmness and stays on the outside, and Yin Qi has the characteristic of softness and emptiness and stays on the inside.