 Finally, the most radical empiricist attitude toward protocol sentences within the Vienna Circle came from Schlick. Schlick endorsed, following Hilbert, a formal, structural notion of communicable, objective knowledge and meaning as well as a correspondence theory of truth. His realism opposed Neurath’s coherentism, and also the pragmatism and conventionalism of Carnap’s Principle of Tolerance in logical matters as well as his Thesis of Metalogic. But like Carnap’s latter thesis and his syntactic approach of 1934, Schlick was concerned with both the Cartesian ideal of foundational certainty and Wittgenstein’s metalinguistic problem of how language represents the reality it is about; such relation could only be shown, not said. In 1934 Schlick proposed to treat the claims motivating protocol statements, left by Neurath with the status of little more than mere hypotheses, as key to the foundation of knowledge. They would be physicalistic statements that, albeit being fallible, could be subjectively linked to statements about immediate private experiences of reality such as ‘blue here now’ he called affirmations (Konstatierungen) (Schlick 1934). Affirmations carried certainty and elucidated what could be showed but not said, they provided the elusive confrontation or correspondence between theoretical propositions and facts of reality. In this sense they afforded, according to Schlick, the fixed starting points and foundation of all knowledge. But the foundation raised a psychological and semantic problem about the acceptance of a protocol. Affirmations, as acts of verification or giving meaning, lacked logical inferential force; in Schlick’s words, they ‘do not occur within science itself, and can neither be derived from scientific propositions, nor the latter from them’ (Schlick 1934, 95). Schlick’s empiricism regarding the role of protocol sentences suggests but does not support strong epistemological foundationalism. Schlick’s occasional references to a correspondence theory of truth were just as unacceptable and were felt to be even more of a philosophical betrayal within the framework of empiricism. Predictably, Neurath rejected Schlick’s doctrines as metaphysical, manifesting the pseudorationalist attitude (Neurath 1934/1983).