The Twofold Mystery School
The Twofold Mystery School was not a sect, but was a Daoist school of thought that based its commentaries to The Book of Dao and its Virtue on the concept of Twofold Mystery, which in turn made it well-known. The origin of the Twofold Mystery was a sentence in chapter one of the Book of Dao and its Virtue: 'the mystery beyond mysteries is the doorway to all subtleties'. The origin of the Twofold Mystery School was related to the Science of Mysteries school of the Wei and Jin dynasties (AD 220-420), when Shun Deng's commentary to the Book of Dao and its Virtue based on the Twofold Mystery, paved the way for its development. After the Wei and Jin dynasties, more Daoist scholars continued in this mystical interpretive tradition. Meanwhile, the Buddhist concept of the Middle Way was absorbed. In the Tang dynasty, the Twofold Mystery school saw its prosperous period when many scholars became its advocates, including the outstanding Daoists Cheng Xuanying and Li Yong. Thanks to their hard work, the Twofold Mystery school became the most influential of Daoist Laozi Learning schools. Even Li Nongji, the Xuanzong Emperor of the Tang dynasty, based himself on the Twofold Mystery School when he commented on Laozi.
The Twofold Mystery's interpretation of Laozi had two characteristics: firstly, ideas from Zhuangzi were quoted in the commentaries; secondly, ideas of Buddhist philosophy, particularly from the Sanguan and Tiantai sects, were quoted in the commentaries. This not only expanded Daoist religious theories but also deepened them.
On the other hand, the Twofold Mystery School had a great influence on Buddhism. For example, the books of the Sanguan sect used the Daoist concepts of 'Mystery' as well as 'Mystery Beyond Mystery" for a long time. Because the Twofold Mystery School focussed on theoretical logic, its scholars often defended Daoism in the polemic between Daoism and Buddhism in the Tang dynasty, which in turn led to the school's achieving high status in Daoism. At the end of the Tang dynasty and during the Five Dynasties (907-960), the impact of the Twofold Mystery School was still felt in ancient Sichuan, although it had been declining as a whole. Even in the Southern Song dynasty, old scholars like Shao Royui and Dong Siqin still understood the Twofold Mystery well, which was also frequently quoted in Daoist works such as the Collection of Commentaries on Laozi of the Song dynasty. This showed the impact left by the Twofold Mystery School.