The Zhouyi cantongui takes the four trigrams, Qian, Kun, Kan and Li, of the book of Changes as the foundation for a system of the cosmos. This system simply describes the workings of the cosmos; there is no account of its origins, no ontologoy. The latter is supplied by the Peng Xiao edition. Whilst the actual practice of alchemy does not require such an account of origins, alchemical thought does and hence the value of the Peng Xiao edition. External alchemists, such as Peng Xiao, believe that the course of alchemy must follow the movements of nature as closely as possible and hence study of nature is required. Internal alchemists, such as the Yu Yan version, whilst not neglecting the natural order, nonetheless stress how this order can be used for spiritual training. The latter seek to make the Way of Heaven become the norm for the human body, viewed as a microcosm.