Maximal functioning means that all kinds of appearances of Dao in the external world are the greatest manifestation and functioning of Dao. Daoist scholars believe that the internal Dao determines the basis for changes in the external world, and that all kinds of forms in the objective world derive from the active, innate nature of Dao, the result of unity of substance and function. In “Twenty- four Styles of Poetry,” Sikong Tu（837-908）, a literary critic in the Tang Dynasty, made this notion a term of literary criticism to highlight the view that the rich and colorful imagery in poetry represents unity of the internal spirit of the work and its external shape. In poetry writing and appreciation, one should focus on the harmony between the appearance and the essence.