 Third, Lewis (1970) insists on a unique interpretation of theoretical terms, thus rejecting Carnap’s doctrine of partial interpretation. Carnap (1961) is most explicit about the indeterminacy that this doctrine implies. This indeterminacy of theoretical terms drives Carnap to using Hilbert’s \(\varepsilon\)-operator there, as just explained. For Lewis, by contrast, a theoretical term is denotationless if its interpretation is not uniquely determined by the Ramsey sentence. For a scientific theory to be true, it must have a unique interpretation.