Neidan, or internal alchemy, is an array of esoteric doctrines and physical, mental, and spiritual practices that Taoist initiates use in hopes to prolong life and create an immortal spiritual body that would survive after death. Also known as Jindan, internal alchemy combines theories derived from external alchemy, correlative cosmology, the emblems of the Yijing, and medical theory, with techniques of Taoist meditation, daoyin gymnastics, and sexual hygiene.