This article is to celebrate the 1000^(th) birthday of Zhang Zai. The "unity of heaven and man," one of the central values of Chinese philosophy, though originating elsewhere, is a thesis proposed by Zhang Zai based on his profound and wide-ranging learning. Here we mean to justify and formulate this thesis with regards to the motivation, purpose and theoretical content of his advocacy of the "unity of heaven and man" by means of his doctrines on Yi Jing (The Book of Changes), Zhuangzi and Zhongyong (The Doctrine of the Mean).