Professor Willem Labuschagne
Computer Science and Information Systems, University of South Africa

Title:  An introduction to nonmonotonic logic and belief revision.

Nonmonotonic logic is concerned with the formalisation of defeasible
reasoning, in which an agent uses a default rule to infer consequences
not necessarily entailed by classical logic. The canonical example is the
inference, from the assumption that Tweety is a bird and the default rule
that birds are normally capable of flying, of the conjecture that Tweety is
able to fly. Such defeasible conjectures may be defeated by additional
information. Should the agent learn that Tweety is an ostrich, the
conjecture that Tweety can fly would have to be retracted.

Belief revision is concerned with sensible ways to adjust the agent's set
of beliefs in the light of new information, and so there should be a
connection with nonmonotonic logic. We establish this link by giving a
semantic interpretation of the AGM postulates for belief revision and
indicating the correspondence with the minimal model semantics of
Shoham, Kraus, Lehmann, and Magidor. The guiding intuition is a simple
generalisation of the notion of semantic information due to Carnap and
Bar-Hillel.

The presentation is elementary, inasmuch as nothing more is assumed
than a vague familiarity with propositional logic and set theory.

When:	1.00 pm, Monday 13 July 

Where:	Seminar Room, 1st Floor, Archway West Building