FLOATING CONCLUSIONS AND ZOMBIE PATHS - 2 DEEP DIFFICULTIES IN THE DIRECTLY SKEPTICAL APPROACH TO DEFEASIBLE INHERITANCE NETS

We discuss two difficulties in the "directly skeptical" approach to inference in defeasible inheritance nets, as developed by Horty, Thomason and Touretzky. We suggest that as a result of the general architecture of the approach, it is intrinsically unable to deal with a phenomenon of "floating conclusions", and has great difficulty in accommodating a phenomenon of "zombi paths". The conclusion drawn is that the directly skeptical approach cannot hope to do the work of an approach via the family of all extensions.