A literal is an atomic sentence or the negation of an atomic sentence. Because they have no arguments, we have no need of object constants, variables, objects, functions, existential quantifiers, explicit universal quantifiers, factors, or even objects.
Unification is trivial.
Relation constants are interpreted by assigning to them a set of n -ary tuples. From the point of view of interpretations, each line in the truth table is an interpretation. Two interpretations are models because they satisfy all the sentences, and two are not. In propositional logic, because we have no variables, every interpretation that satisfies the set of sentences is by definition a model. This idea is known as tautological entailment .