Floating Conclusions and Zombie Paths: Two Deep Difficulties in the "Directly Skeptical" Approach to Defeasible Inheritance Nets

Makinson, D. and K. Schlechta, Floating conclusions and zombie paths: two deep difficulties in the "directly skeptical" approach to defeasible inheritance nets (Research Note), Artificial Intelligence 48 (1991) 199-209. 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.