There are various dynamics in Chinese cosmology. In the cosmology pertaining to Yang and Yin, the material energy, which this universe has created itself out of, is also referred to as qi. It is believed that the organization of qi in this cosmology of Yin and Yang has formed many things. Included among these forms are humans. Many natural dualities (such as light and dark, fire and water, expanding and contracting) are thought of as physical manifestations of the duality symbolized by yin and yang. This duality lies at the origins of many branches of classical Chinese science and philosophy, as well as being a primary guideline of traditional Chinese medicine, and a central principle of different forms of Chinese martial arts and exercise, such as baguazhang, taijiquan (t'ai chi), and qigong (Chi Kung), as well as appearing in the pages of the "I Ching".