 Objects exist in time, and they change their properties over time. Propositional temporal logics are not expressive enough to capture this dynamic aspect of the world, for all that is associated with a time instant in a model of propositional temporal logics is a set of abstract atomic propositions that are declared true there. What is needed, instead, is a full-fledged model of the temporal history of the world, with objects that may have certain properties and stand in certain relations. Accordingly, the language should contain names for objects, variables and quantifiers ranging over objects, as well as predicates for denoting properties and relations, to adequately describe how the world is at a given instant in time — in addition to temporal operators for reasoning about how the world changes over time. This is what first-order temporal logics provide.