Abstract: The use of nonmonotonic or "common sense" reasoning has often been advocated to speed up reasoning systems. The difficulty has always been that nonmonotonic reasoning requires consistency checks, which are even more computationally intractable than the monotonic reasoning normally used in reasoning systems. Disclosed herein are methods and apparatus for making consistency checks tractable by performing them over a limited context using fast sufficient consistency checks. The results are approximate, but approximate results are expected in nonmonotonic reasoning. Included in the disclosure are techniques for selecting the limited context and making the fast sufficient consistency checks and a default reasoner which employs the techniques to check the consistency of a default conclusion with a limited context.