The most original contribution of Chang Tsai (1020-1077) to Chinese philosophy is his concept of "Ch'i" or vital force. The three main points discussed in this article are as follows：I) the meaning and characteristics of "Ch'i," which is, according to Chang，the fundamental substance by which all processes of the universe can be explained. II) the undifferentiation of the vital force and the great void. Although Chang accepted the Taoist-Buddhist term "great void" and used it frequently，his connotation of it is quite different from what is meant by the Taoists and the Buddhists. III) Chang's realism versus Buddhist nihilist metaphysics. Chang's concept of "Ch'i" serves for him the double purpose of attacking the Buddhist theory of reality on the one hand and of constructing a sounder one of his own on the other.
