 Alchemy developed in China in two main branches. Waidan, or External Alchemy, is based on compounding elixirs through the manipulation of natural substances—primarily minerals and metals—which release their essences when they are submitted to the action of fire. Neidan, or Internal Alchemy, aims to produce the elixir within the alchemist’s person according to two main models of doctrine and practice: first, by causing the primary components of the cosmos and the human being—Essence, Breath, and Spirit—to revert to their original states; and second, by purifying one’s mind of defilements and passions, in order to “see one’s Nature” (jianxing), which is equated to the elixir.