Cut-Based Abduction

In this paper we explore a generalization of traditional abduction which as simultaneously perform two different tasks: (i) given an unprovable sequent Gamma proves G, find a sentence II such that Gamma, II proves G is provable (hypothesis generation); (ii) given a provable sequent Gamma proves G, find a sentence II such that Gamma proves II and the proof of Gamma, II proves G is simpler than the proof of Gamma proves G (lemma generation). We argue that the two tasks should not be distinguished, and present a general procedure for finding suitable hypotheses or lemmas. When the original sequent is provable, the abduced formula can be seen as a cut formula with respect to Gentzen's sequent calculus, so the abduction method is cut-based. Our method is based on the tablean-like system KE and we argue for its advantages over existing adduction methods based on traditional Smullyan-style Tableaux.