Becoming an immortal can be classified into ghost immortals, human immortals, divine immortals, loose immortals, earth immortals, and heavenly immortals. It generally refers to the process of training oneself through some means and constantly transforming oneself to become an immortal. This is the art of refining elixirs and taking medicine, or to calm the mind and nourish the mind, or to refine the heaven and enlighten the Tao, in order to achieve one's own strength and immortality.