Cheng Xuanying, alias Zi Shi, was a Daoist in the early time of the Tang dynasty (618-907), and he was an outstanding Daoist scholar at the time. Cheng was born in Shan Zhou (in today's Shan County of Henan province) and he once lived in seclusion on the coast of the eastern sea. In the 5th year of Zhenguan (631AD), the first emperor of the Tang invited him to the capital and gave him the title "Master of West China". In the Yonghui (650-655AD) year of the Tang, Cheng was exiled to Yu Zhou (in today's Mount Yun Tai of Jiangsu province). He mastered the Laozi and Zhuangzi and authored the Glosses and Comments on the Preface to the Interpretations of the Title of the Book of Dao and its Virtue ( 《老子道德經開題序訣義疏》 Laozi Daodejing Kaiti ) and the Commentary on the Perfect Book of Nanhua ( 《南華真經疏》 Nanhua Zhenjing Shu ). In his glosses and comments he mainly elucidated the thought of the "Twofold Mystery" school so that the philosophy of the Twofold Mystery became the main current of Daoist philosophy in the early years of the Tang.