Nonmonotonic Reasoning as a Temporal Activity
  A {\it dynamic reasoning system} (DRS) is an adaptation of a conventional
formal logical system that explicitly portrays reasoning as a temporal
activity, with each extralogical input to the system and each inference rule
application being viewed as occurring at a distinct time step. Every DRS
incorporates some well-defined logic together with a controller that serves to
guide the reasoning process in response to user inputs. Logics are generic,
whereas controllers are application-specific. Every controller does,
nonetheless, provide an algorithm for nonmonotonic belief revision. The general
notion of a DRS comprises a framework within which one can formulate the logic
and algorithms for a given application and prove that the algorithms are
correct, i.e., that they serve to (i) derive all salient information and (ii)
preserve the consistency of the belief set. This paper illustrates the idea
with ordinary first-order predicate calculus, suitably modified for the present
purpose, and an example. The example revisits some classic nonmonotonic
reasoning puzzles (Opus the Penguin, Nixon Diamond) and shows how these can be
resolved in the context of a DRS, using an expanded version of first-order
logic that incorporates typed predicate symbols. All concepts are rigorously
defined and effectively computable, thereby providing the foundation for a
future software implementation.
