The word "Daoqi" (道氣) is a concept Daoists developed in the Han Dynasty (206 BCE-220 CE) that emphasizes the "Dao" as having a substance-based ontology. Most significantly, this notion of Dao as substance implies the connection of the metaphysical and physical realms. During the Six Dynasties (220-589), "Daoqi" became a popular term and was commonly used in Daoist texts. In these works, it had a range of six different meanings: the foundational origin on which all humans and things rely; the essence of the gods and ghosts; a mystical power beyond comprehension; a prerequisite for cultivation and the verification of its success; a special natural endowment; an indicator that the utopian age of a "Dao realized world" is approaching. During the Tang Dynasty (618-907), "Daoqi" developed into a word that often appeared in works of the literati with varying meanings. However, in Daoism, it has always been tied to the argument about whether "Dao exits prior to qi" or "Dao is just qi." Since the Song Dynasty (960-1279), the idiomatic expression "May your Daoqi last forever" (道氣常/長存) became a common way to convey blessings or good wishes.