Showing papers by "Paul Morris published in 1987"

Curing anomalous extensions

Abstract: In a recent paper, Hanks and McDermott presented a simple problem in temporal reasoning which showed that a seemingly natural representation of a frame axiom in non monotonic logic can give rise to an anomalous extension, i.e., one which is counter-intuitive in that it does not appear to be supported by the known facts. An alternative, less formal approach to nonmonotonic reasoning uses the mechanism of a truth maintenance system (TMS). Surprisingly, when reformulated in terms of a TMS, the anomalous extension noted by Hanks and McDermott disappears. We analyze the reasons for this. First it is seen that anomalous extensions are not limited to temporal reasoning, but can occur in simple non-temporal default reasoning as well. In these cases also, the natural TMS representation avoids the problem. Exploring further, it is observed that the form of the TMS justifications resembles that of nonnormal default rules. Nonnormal rules have already been proposed as a means of avoiding anomalous extensions in some non-temporal reasoning situations. It appears that, suitably formulated, they can exclude the anomalous extension in the Hanks-McDermott case also, although the representation does not adjust smoothly to fresh information, as does the TMS. Some variant of nonnormal default appears to be required to provide a correct semantic basis for truth maintenance systems.

A Truth Maintenance Based Approach To The Frame Problem

Abstract: We explore an approach to the frame problem that employs nonmonotonic justifications, using the inferential machinery of a truth maintenance system (TMS). This approach avoids the anomalous extension noted by Hanks and McDermott. We analyze the reasons for this. First it is seen that the Hanks-McDermott difficulty is not limited to temporal reasoning, but occurs also in simple nontemporal default reasoning. It is noted that the natural TMS representation cures such problems. Exploring further, it is observed that nonnormal default rules are required to simulate the behavior of the TMS. It appears that such rules provide a general mechanism for overcoming the Hanks-McDermott difficulty.