The Subsumption Theorem in Inductive Logic Programming: Facts and Fallacies

The subsumption theorem is an important theorem concerning resolution. Essentially, it says that if a set of clauses logically implies a clause C, then either C is a tautology, or a clause D which subsumes C can be derived from with resolution. It was originally proved in 1967 by Lee. In Inductive Logic Programming, interest in this theorem is increasing since its independent rediscovery by Bain and Muggleton. It provides a quite natural \bridge" between subsumption and logical implication. Unfortunately, a correct formulation and proof of the subsumption theorem are not available. It is not clear which forms of resolution are allowed. In fact, at least one of the current forms of this theorem is false. This causes a lot of confusion. In this paper we give a careful proof of the subsumption theorem for unconstrained resolution, and show that the well-known refutation-completeness of resolution is just a special case of this theorem. We also show that the subsumption theorem does not hold when only input resolution is used, not even in case contains only one clause. Since some recent articles assume the contrary, certain results (for instance results on nth roots and nth powers) in these articles should perhaps be reconsidered.