 Of course, even if descriptions are not directly referential, some are rigid designators. For example, ‘the successor of 1’ designates the same individual (2) in all possible worlds. So, though all directly referential expressions are rigid designators, some rigid designators are not directly referential. As was mentioned above, in a possible worlds semantics linguistic expressions are associated with intensions, functions from possible worlds to appropriate extensions. In the case of expressions designating individuals, these intensions will be functions from possible worlds to individuals. Note that all rigid designators (whether directly referential or not) will have intensions that are constant functions: they will be functions that map all possible worlds to the same individual. Thus possible worlds semantics tends to blur the distinction between directly referential expressions and rigid non-directly referential expressions (e.g. rigid definite descriptions). To make the distinction between directly referential expressions and rigid non-directly referential expressions more vivid, Kaplan invoked the notion of structured propositions: