 In his paper “Theories” (1929a), written in the summer of 1929 (Braithwaite 1931, xiii), Ramsey calls the observational language ‘primary system’. The non-observational (theoretical) language is called ‘secondary system’. The paper is thoroughly instrumentalistic, since the factual content of theories consists only of their primary implications. This is made clear in a comment on the paper, written in August 1929 ([NP] 229, footnote removed; see also Ramsey (1931, [FM] 260):