Book excerpt: Available for the first time in 20 years, here is the Rudolf Carnap's famous "principle of tolerance" by which everyone is free to mix and match the rules of language and logic. In The Logical Syntax of Language, Carnap explains how his entire theory of language structure came to him like a vision when he was ill. He postulates that concepts of the theory of logic are purely syntactical and therefore can be formulated in logical syntax.