Rudolf Carnap's entire theory of Language structure came to me, he reports, like a vision during a sleepless night in January 1931, when I was ill. This theory appeared in The Logical Syntax of Language (1934). Carnap argued that many philosophical controversies really depend upon whether a particular language form should be used. This leads him to his famous Principle of tolerance by which everyone is free to mix and match the rules of his language and therefore his logic in any way he wishes. In this way, philosophical issues become reduced to a discussion of syntactical properties, plus reasons of practical convenience for preferring one form of language to another. In a tour de force of precise reasoning, Carnap also indicated how two model languages could be constructed. This is one of three books which Open Court is making available in paperback reprint in its Open Court Classics series. The other two are Carnap's The Logical Structure of the World and Schlick's General Theory of Knowledge.