  The term “Twofold Mystery” comes from chapter 1 of the Laozi, where Dao is said to be the mystery of all mysteries, in the sense of an incomparably profound reality beyond ordinary comprehension (xuan zhi you xuan). As a school of Daoist learning, “Twofold Mystery” seizes this to be the key to understanding the Laozi. Daoist sources relate that the school goes back to the fourth-century master Sun Deng. Through Gu Huan (fifth century) and others, the school reached its height during the Tang period, represented by such thinkers as Cheng Xuanying and Li Rong in the seventh century. The school reflects the growing interaction between Daoist and Buddhist thought, particularly Mādhyamika philosophy. Unlike Wang Bi, it sees “nonbeing” as equally one-sided as “being” when applied to the transcendence of Dao. Nonbeing may highlight the profundity or mystery of Dao, but it does not yet reach the highest truth, which according to Cheng Xuanying can be called the “Dao of Middle Oneness” (Kohn 1992, 144; Assandri 2009, chs. 4 and 6). Like other polar opposites, the distinction between being and nonbeing must also be “forgotten” before one can achieve union with Dao.