Given a data base of facts (ground instances) and rules expressed in logic, we can pose questions in logic and answer them using resolution.

Example:

1. &forall x &forall y &forall z [Parent(x,z) &and Parent(z,y) &rarr Grandparent(x,y)]
2. &forall x &forall y [Father(x,y) &rarr Parent(x,y)]
3. &forall x &forall y [Mother(x,y) &rarr Parent(x,y)]
4. Father(Zeus, Ares)
5. Mother(Hera, Ares)
6. Father(Ares, Harmonia)

Consider the question, ``Who is a grandparent of Harmonia?'' This can be expressed as &exist x Grandparent(x, Harmonia) ; this is negated to be &forall x ¬ Grandparent(x, Harmonia) (``Nobody is the grandparent of Harmonia.'').

We then can use resolution to derive □, thus proving that Harmonia does have a grandparent. But who is it?

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