"Theory of Language" is the last monograph left by Mr. Gao Mingkai for future generations, and it is the result of his years of research. The book is divided into three parts: The first part discusses the social nature of language, the functions of language, the communicative function of language, the relationship between language and thought, the distinction between language and speech, and the varieties of language; the second part discusses the symbolic nature of language, its systematic nature, the systems and structures of language's phonetics, semantics, vocabulary and grammar; the third part discusses the origin of language, the development of language, the differentiation of language, the unification of language and the prospects for the development of language, etc.