This article takes a clear-cut case in which a historian (Alexander Koyré) ascribes to a writer (Galileo) a concept ("inertial mass") which neither the writer nor his contemporaries had the linguistic means to express. On the face of it the case may seem a violation of a basic methodological maxim in historiography: "avoid anachronistic ascriptions!" The aim of the article is to show that Koyré's ascription, and others of its kind, are legitimate; and that the methodological maxim should not be given the strict reading which some writers recommend. More specifically, the conceptual repertoire of historical figures need not be reconstructed solely in terms of the social and linguistic conventions of their time and place.