The period of the Zhou Dynasty and its decline is commonly taken as a time in which Chinese thought gradually moved toward more and more humanistic and rationalistic understandings of the world and the place of human beings in it. Around the third century b.c.e., China experienced a particular shift from a mystical–religious view of the operations of the world toward naturalistic analyses of specific causes. The rise of yinyang thinking and terminology at that time is intimately connected with this shift, as yinyang became an important conceptual tool to facilitate this transition. This shift has analogies with the step from mythos to logos in the ancient Mediterranean world, and yinyang thinking functions in ways similar to the recognition of natural laws and the reduction of phenomena to elements. It advocates a rational effort to furnish an intellectual and coherent account of the natural world and the human condition. Taking a phrase from Paul Unschuld, we can say that this period was directed toward increasing “existential autonomy,” allowing human beings to take more and more control over their own lives. Yinyang became the most effective and multifarious concrete conceptual tool for understanding the human body.

The theorization of the human body as a yinyang construct through the systematization of Chinese medicine is the best illustration of the development of yinyang thought and practice. The Huangdi Neijing states simply, “A good practitioner differentiates between yin and yang when observing the complexion and feeling the pulse.” This view is different from earlier beliefs that illness was either a curse from the ancestors or some kind of punishment. In Shang oracle inscriptions that have been found on bone and turtle shell, it is demons and the spirits of the dead that sicken the bodies of the living. Harper clarifies this view, writing, “Demonic illness reflects the belief that something with an existence outside the body has relocated on or in the body; exorcism is a logical treatment.”