 Insofar as the empirical adequacy of a theory amounts to the embeddability of observable phenomena within substructures of the theory’s models, the constructive empiricist’s account of empirical adequacy rests heavily on the distinction between what is observable and what is not. If, as it is natural to think,‘is observable’ is a vague predicate, we should not expect there to be a precise demarcation between what’s observable and what’s unobservable. Observability can still serve as a useful concept in the philosophy of science, as long as there are clear cases of observability and clear cases of unobservability.