In first-order logic with identity, the sentence ∀x(x = x) is a ground formula. A ground expression is a ground term or ground formula.
Consider the following expressions in first order logic over a signature containing a constant symbol 0 for the number 0, a unary function symbol s for the successor function and a binary function symbol + for addition.
- s(0), s(s(0)), s(s(s(0))), ... are ground terms,
- 0 + 1, 0 + 1 + 1, ... are ground terms,
- x + s(1) and s(x) are terms, but not ground terms,
- s(0) = 1 and 0 + 0 = 0 are ground formulae,
- s(1) and ∀x: (s(x) + 1 = s(s(x))) are ground expressions.
What follows is a formal definition for first-order languages. Let a first-order language be given, with C the set of constant symbols, V the set of (individual) variables, F the set of functional operators, and P the set of predicate symbols.
Ground terms are terms that contain no variables. They may be defined by logical recursion (formula-recursion):
- Elements of C are ground terms;
- If f ∈ F is an n-ary function symbol and α1, α2, ..., αn are ground terms, then f(α1, α2, ..., αn) is a ground term.
- Every ground term can be given by a finite application of the above two rules (there are no other ground terms; in particular, predicates cannot be ground terms).
Roughly speaking, the Herbrand universe is the set of all ground terms.
A ground predicate, ground atom or ground literal is an atomic formula all of whose argument terms are ground terms.
If p ∈ P is an n-ary predicate symbol and α1, α2, ..., αn are ground terms, then p(α1, α2, ..., αn) is a ground predicate or ground atom.
A ground formula or ground clause is a formula without free variables.
Formulas with free variables may be defined by syntactic recursion as follows:
- The free variables of an unground atom are all variables occurring in it.
- The free variables of ¬p are the same as those of p. The free variables of p∨q, p∧q, p→q are those free variables of p or free variables of q.
- The free variables of ∀x p and ∃x p are the free variables of p except x.