 On the basis of these principles, the only form of alchemical practice sanctioned by the Cantong qi and by the works that follow its system is the one that enables the conjunction of Qian and Kun, or True Yang and True Yin. According to the Cantong qi, only True Lead and True Mercury are “of the same kind” (tonglei) as Qian and Kun. The Yin and Yang entities that contain these authentic principles are “black lead” (i.e., native lead) and cinnabar, respectively. In the strict sense of the term, alchemy therefore consists in extracting True Lead from “black lead” and True Mercury from cinnabar, and in joining them to one another. This is accomplished in Waidan by using the corresponding minerals and metals, and in Neidan by operating on various components of the human body—both physical and non-physical—which are symbolically denoted by the same terms.