 Carnap’s first sketch of a linguistic framework (that of the Aufbau) envisaged complete reducibility of all concepts by explicit definition, at least in principle. By the time of “Testability and Meaning” (1936–37), as we saw above, Carnap had conceded that such definitional reducibility is not always possible, and suggested various forms of incomplete reducibility that, while not employing only explicit definitions, at least required of empirically meaningful sentences a clear grounding in observation. He considers verifiability as a criterion for empirical content or meaning, and rejects it for essentially the same reason as he had a decade previously (in Carnap 1926: 7–9). He also considers Popper’s falsifiability criterion, and rejects it because only sentences of a very particular logical form are falsifiable at all (such as, e.g., “all swans are white”). Indeed, most scientific theories (all theories that use limit concepts, for instance) have to be stated using unbounded multiple mixed quantifiers, e.g., “for all x there is a y such that \((\ldots x \ldots y\ldots)\)”, and there are instances of such sentences which can neither be verified nor falsified by a single observational counter-instance. In particular, to falsify such a statement there would have to be a counter-instance x to falsify an unbounded existentially quantified (empirical) sentence, which (as Popper himself acknowledged) is not generally possible. So in these cases, typical of theoretical science, falsifiability has no advantage over verifiability. (For a more precise discussion of falsifiability, see Schurz &amp; Dorn 1988.)