Zhu Xi, the twelfth-century architect of the neo-Confucian canon, declared Zhou Dunyi to be the first true sage since Mencius. This was controversial, as many of Zhu Xi's contemporaries were critical of Zhou Dunyi's Daoist leanings, and other figures had clearly been more significant to the Song dynasty Confucian resurgence. Why was Zhou Dunyi accorded such importance? Joseph A. Adler finds that the earlier thinker provided an underpinning for Zhu Xi's religious practice. Zhou Dunyi's theory of the interpenetration of activity and stillness allowed Zhu Xi to proclaim that his own theory of mental and spiritual cultivation mirrored the fundamental principle immanent in the natural world. This book revives Zhu Xi as a religious thinker, challenging longstanding characterizations of him. Readers will appreciate the inclusion of complete translations of Zhou Dunyi's major texts, Zhu Xi's published commentaries, and other primary source material.