In order to understand the way that TCM looked at female bodies, it is critical to understand the relationship of "yin" and "yang" to the practice of medicine. According to Charlotte Furth and her book, "A Flourishing Yin: Gender in China’s Medical History", yin and yang showed the embodiment of nature within human bodies, and with it the natural phenomena that we understand as gendered differences. Yin and yang ruled the body, the body being a microcosm of the universe and the earth. In addition, gender in the body was understood as homologous, the two genders operating in synchronization.