 The charge of a constitutive failing would rather seem to come from Demopoulos’s challenge to the two-languages model (nearly) universally adopted in logical empiricism (forthcoming). Importantly, this challenge does not proceed, as some previous ones have, from the impossibility of drawing a sharp distinction between the observational and the theoretical (Putnam 1962). Rather, the two-languages model falsely supposes that the process of testing scientific hypotheses must only advert to theoretically uncontaminated facts and so results in misunderstanding the empirical import of theoretical claims (as in the Newman problem). Instead, a conception of theory-mediated measurement and testing is suggested that extends responsiveness to observational data to theoretical claims by showing them to be essentially implicated in the production of observed experimental consequences. Hoping to advance beyond the stalemate between realism and instrumentalism without appealing to question-begging semantics, Demopoulos here breaks with a supposition upheld by Carnap throughout, namely that the theoretical language be regarded as an essentially uninterpreted, at best partially interpreted calculus. Whatever the outcome of this challenge, it is remarkable how on this far-reaching and fundamental issue contemporary philosophy of science intersects with the history of philosophy of science.