 The simplest example of this phenomenon is given by the relationship between the two best-known logical languages, the propositional and the first-order predicate one, when used to describe first-order models. The validity problem for the propositional language, which can be understood as one that only allows us to talk about the properties (i.e., monadic predicates) of a single object and their Boolean combination, is decidable: there are effective procedures that can answer the validity question for any given propositional formula. The first-order predicate language can see much more (all objects of the domain, together with their properties and their n-ary relations, among others), but this comes at a price: its validity problem is undecidable, as there is no effective procedure that can answer the validity question for all its formulas.