Pretheoretically, there is a fundamental tension in the ordinary language term concept. On one hand, it is something out there in the world: We speak of “the Newtonian concept of mass” as though it exists independently of who actually knows or grasps it. Likewise, “grasping a concept” evokes comparison to grasping a physical object, except that we somehow do it with our minds instead of our hands. On the other hand, we speak of a concept as an entity within one's head, a private entity, a product of the imagination that can be conveyed to others only by means of language, gesture, drawing, or some other imperfect means of communication. How should a theory of concepts confront this tension?