  Ramsey already discussed Weyl earlier, for example in ‘Mathematical Logic’ (1926c, [PP] 230 &amp; 233), without approving him, so one needs to explain why he changed his mind in 1928–29. This may be the result of changes in his underlying conception of theories from his 1928 manuscript On Truth ([OT] 33–34) to ‘Theories’ (1929). According to his later view, a ‘primary system’ of true or false observational statements is entailed, via a ‘dictionary’, by a ‘secondary system’ of hypotheses (1929a, [PP] 112–115). This is a view that has roots in both the introduction to Heinrich Hertz’s The Principles of Mechanics (Hertz 1899) and in chapter VI of Norman Campbell’s Physics. The Elements (Campbell 1920), but Ramsey described, as we shall see, hypotheses not as statements or propositions, but, adapting Weyl’s expression, as ‘rules for judging’.