 In the semantics outlined above, variable assignments are defined globally, and hence individual variables pick out the same object at all times. The interpretation of constant symbols, on the other hand, is specified locally, relative to a time instant, even though the respective extensions range over the global domain. From a philosophical point of view, it seems natural to treat constant symbols as proper names or as rigid designators, that is, to impose the additional requirement that their interpretations be constant across different times. Constant symbols can then be used to identify an object in time. For instance, treating \(a\) as a name for Aristotle, the sentence “Aristotle existed but does no longer exist” can be formalized in the varying domain semantics as \(P \exists x(x=a) \wedge \neg\exists x(x=a)\).