Internal alchemy arose within Daoism from around the Tang dynasty and came to prominence as the external, mineral alchemy declined, discredited in part because its mercury- and lead-based elixirs of immortality were, in fact, lethal. The internal practice reinterpreted the whole alchemical vocabulary as a map of inner processes, so that the ingredients, reactions and stages of the alchemical texts became coded instructions for meditation and energetic cultivation, guarded by deliberately obscure symbolic language that required a master's oral transmission to unlock.
Neidan became central to the great monastic Daoist movements, especially the Complete Reality (Quanzhen) school that arose in the twelfth century and remains a principal Daoist tradition. Its techniques overlap with the wider field of qigong and Daoist "nourishing life" practices, including the circulation of energy known as the microcosmic orbit, and its detailed inner cartography of channels, energy-centres and refinement stands as the Daoist counterpart to the subtle-body yogas of the Indian traditions, arrived at independently and framed throughout by the Daoist quest to return to the source of all things.