latter's contention that an emperor in 60 BCE had hired "a well-known scholar, Liu Hsiang, as Master of the Recipes so that he could make alchemical gold and prolong the Emperor's life." All of these conflicting origins considered, it is nearly impossible to claim any absolute knowledge on the origins of Chinese alchemy. However, historical texts of Daoist teaching include alchemical practices, most of which posit the existence of an elixir or the Golden Elixir that, when ingested, gives the drinker eternal life. As there is a direct connection between Daoism and Laozi, some suggest he played a major role in the creation of Chinese alchemy. Zhou Dynasty philosopher Zou Yan is said to have written many of the alchemical books, although none of them have ever been found, nor have the existing ones been credited to him. The likeliest proponents of Chinese alchemy are as previously stated, Laozi, and Zhang Daoling as well as Zhuangzi. Each of these men are major icons in Daoist teachings. Although these three are credited with the creation of alchemy, there is no definitive proof to suggest or dispute that they were responsible for its creation. Yin and Yang The concept of yin-yang is pervasive throughout Chinese alchemical theory. Metals were categorized as being male or female, and mercury and sulphur especially were thought to have powers relating to lunar and solar respectively. Prior to Taoist tradition, the Chinese already had very definitive notions of the natural world's processes and "changes", especially involving the wu xing: Water, Fire, Earth, Metal and Wood. These were commonly thought to be interchangeable with one another; each were capable of becoming another element. The concept is integral, as the belief in outer alchemy necessitates the belief in natural elements being able to change into others. The cyclical balance of the elements relates to the binary opposition of yin-yang, and so it appears quite frequently. Outer and inner alchemy Chinese alchemy can be divided into two methods of practice, waidan or "external alchemy" and neidan or "internal alchemy". Doctrine can be accessed to describe these methods in greater detail; the majority of Chinese alchemical sources can be found in the Daozang, the "Taoist Canon". Outer alchemy (Waidan) The meaning of waidan derives from wai (outside, exterior) and dan referring to alchemical operations, such as the preparation of chemical elixirs, made from cinnabar, realgar, and other substances generally involving mercury,