There are two important theorems known as Herbrand's theorem.
The Herbrand theorem in logic states that a formula is unsatisfiable iff there is a finite set of ground clauses of that is unsatisfiable in propositional calculus. It is assumed that elements of the Herbrand base are treated as propositional variables. Since unsatisfiability is dual to validity ( is unsatisfiable iff the negation is valid), the Herbrand theorem establishes that the Herbrand universe alone is sufficient for interpretation of first-order logic. This theorem also reduces the question of unsatisfiability in first-order logic to the question of unsatisfiability in propositional calculus.