 This is a question to which there are many rival answers, even for the most basic logical constants. Perhaps the expressions must denote the same truth-function, or have the same intension, or share a mode of presentation, or a character, or a conceptual role. But The Logical Syntax of Language was published (in German) in 1934, before the innovations of Grice, Gentzen, Montague, Kaplan, Lewis, Putnam or Kripke, (and, moreover before Tarski’s “On the Concept of Logical Consequence” (Schurz, 1998; Tarski 1983)) and in an environment in which Wittgenstein’s Tractatus Logico-Philosophicus was a powerful influence. Carnap has quite definite and explicit ideas about both meaning and logic, and these help to explain why he thinks linguistic tolerance leads straight to logical tolerance. In the foreword he writes: 