Neidan, "internal alchemy," is the Daoist contemplative discipline of refining the vital energies of the body to attain health, longevity and, in its highest aim, spiritual transcendence and union with the Dao. It takes the language, imagery and stages of the older laboratory alchemy, the crucible, the furnace, the refining of an elixir, and turns them inward, treating the human body itself as the vessel in which the elixir of immortality is compounded. Where the external alchemists sought to concoct a physical elixir from minerals and metals, the internal alchemists work with the body's own "three treasures": jing (essence), qi (vital energy) and shen (spirit), refining and transmuting one into the next through meditation, breath and visualisation.
The classic sequence describes the transformation of essence into energy, energy into spirit, and spirit back into the emptiness of the Dao, a progressive sublimation by which the adept is said to build within the body a spiritual embryo, an immortal self that survives the death of the physical frame.