 It may be objected that every concept must have an extension, that there must be a definite totality of objects that fall under the concept. If this is right then a predicate is meaningful—it expresses a concept—only if the predicate necessarily demarcates the world sharply into those objects to which it applies and those to which it does not apply. Hence, the objection concludes, no predicate with an essentially circular definition can be meaningful. The objection is plainly not decisive, for it rests on a premiss that rules out many ordinary and apparently meaningful predicates (e.g., ‘bald’). Nonetheless, it is noteworthy because it illustrates how general issues about meaning and concepts enter the debate on the requirements on legitimate definitions.