 We may also drop the assumption that singular terms are rigid designators and thus allow nonrigid designators. On the semantics considered here, these are singular terms \(t\) such that for some worlds \(w_1\) and \(w_2, \bI(t,w_1) \ne \bI(t,w_2)\). Definite descriptions, understood attributively, are the best examples. Thus the description “the oldest person” designates different people at different times (worlds)—and no one at times before people existed (“worlds” \(w\) at which \(\bI(t,w)\) is undefined).