Talk by Hector Levesque:
The Truth about Defaults
Date : Monday, Oct 2nd
Time : 3pm
Room : PT 266
Abstract
Virtually all of the work on defaults in AI has concentrated on default
reasoning: given a theory T containing facts and defaults of some sort, we
study how an ideal agent should reason with T, typically in terms of
constructs like fixpoints, or partial orders, or nonmonotonic entailment
relationships. In this talk, we investigate a different question: under what
conditions should we consider the theory T to be true, or believed to be
true, or all that is believed to be true? By building on truth in this way,
we end up with a logic of defaults that is classical, that is, a logic with
an ordinary monotonic notion of entailment. And yet default reasoning
emerges naturally from these ideas. We will show how to characterize the
default logic of Reiter and the autoepistemic logic of Moore in purely
truth-theoretic terms. We will see that the variant proposed by Konolige is
in fact a link between the two, and that all three fit comfortably within a
single logical language, that we call O3L. Finally, we will present first
steps towards a proof theory (with axioms and rules of inference) for O3L.
Among other things, this allows us to present ordinary sentence-by-sentence
derivations that correspond to different sorts of default reasoning.
This is joint work with Gerhard Lakemeyer.
