 Propositional logic is decidable but first-order logic, many-sorted version included, is undecidable. However, in between propositional and first-order logic there are decidable logics, like monadic predicate logic (first-order logic whose predicates are all unary predicates), as well as decidable  fragments[10]  of the undecidable logic. Moreover, among the decidable problems there are degrees of time-space complexity measuring time and memory register used by the computer.