 Bridgman reminded us forcefully that measurement operations did not have unlimited domains of application, and that our conceptual structures consequently had “joints” at which operational meanings changed. But there can be no “joints” if there is no continuous tissue around the disjointed bones. Less metaphorically: if we reduce meaning entirely to measurement operations, there are no possible grounds for assuming or demanding any continuity of meaning where there is clear discontinuity in measurement operations. When we have two different operations which give convergent results in the overlapping domain, how do we tell whether what we have is an accidental convergence of the measured values of two unrelated quantities, or a unified concept measured by two different methods? Some critics have maintained that only a recourse to theories can give us the answer (e.g., Lindsay 1937, 458; Gillies 1972, 23). That does not seem to me always necessary, as theory is not the only source of semantic continuity. There are instrumental operations that are not metrological, and these operations can provide a continuity of meaning against which metrological validity can be judged (see Section 2.1). We can take operationalism as a useful and practicable caution not to make conceptual extensions without operational grounds.