In the early 1930s, Carnap’s philosophical programme underwent a radical change of method, which mainly consisted of two successive breakthroughs. These have been carefully distinguished by Steve Awodey and Andre Carus, who have also fully articulated the historical and philosophical context in which they occurred.1 The first step was taken in January 1931 during a sleepless night when ‘the whole theory of language structure and its possible applications in philosophy came to [Carnap] like a vision’ (Carnap 1963a, p. 53), a new theory which ingeniously combined an axiomatic (Hilbert-style) approach to language and a purely formal syntactical method of analysis, worked out from a standpoint that Carnap then called ‘metalogic’. At that time, it was far from clear that the full development of this method required a different essentially richer metalanguage, a point that Carnap would nevertheless have to acknowledge before long. This first step was then followed by a second one, taken in all likelihood in October 1932, when he adopted the principle of tolerance, an idea which he later explained in more detail. In The Logical Syntax of Language (1934), this principle is stated in the often quoted following terms: ‘In logic there are no morals. Everyone is at liberty to build up his own logic, i.e. his own form of language, as he wishes’ (Carnap 1934d/1937, §17). As a result of these two breakthroughs, from then on and through the rest of his philosophical career, Carnap kept two main ideas as key elements of his method. The first one is the explicit distinction between a language and a metalanguage, with the adoption of a metalinguistic viewpoint for what he called the ‘logic of science’, with which he proposed to replace traditional philosophy.