  Theories of qi and yin and yang also are importantly pursued in early philosophical works. Several Warring States texts contain references to the nature of qi, yin and yang, and their relation to health and longevity. The Zhuangzi clearly identifies qi as the basis of the physical constitution of the body: “Human birth is caused by the gathering together of qi” (22, 733, misleadingly translated by Watson (2013, 180) as “purity”). The Zhuangzi also describes harmonizing or taking charge of the six qi.