On the intertranslatability of non-monotonic logics

• Authors: Tomi Janhunen
• Venue: Annals of Mathematics and Artificial Intelligence
• Year: 1999

This paper concentrates on comparing the expressive powers of five non‐monotonic logics that have appeared in the literature. For this purpose, the concept of a polynomial, faithful and modular (PFM) translation function is adopted from earlier work by Gottlob, but a weaker notion of faithfulness is proposed. The existence of a PFM translation function from one non‐monotonic logic to another is interpreted to indicate that the latter logic is capable of expressing everything that the former logic does. Several translation functions are presented in the paper and shown to be PFM. Moreover, it is shown that PFM translation functions are impossible in certain cases, which indicates that the expressive powers of the logics involved differ strictly. The comparisons made in terms of PFM translation functions give rise to an exact classification of non‐monotonic logics, which is then named as the expressive power hierarchy (EPH) of non‐monotonic logics. Three syntactically restricted variants of default logic are also analyzed, and EPH is refined accordingly. Most importantly, the classes of EPH indicate some astonishing relationships in light of earlier results on the expressive power of non‐monotonic logics presented by Gottlob as well as Bonatti and Eiter: Moore’s autoepistemic logic and prerequisite‐free default logic are of equal expressive power and less expressive than Reiter’s default logic and Marek and Truszczyński’s strong autoepistemic logic.