Against Nonmonotonic Logic (DRAFT)

The monotonicity property of inference in conventional formal systems of logic (S ` A and S ⊆ T implies T ` A) appears to prohibit certain crucial steps in practical intuitive reasoning. For example, reasoning by default (using assertions that are assumed to hold until explicitly contradicted) appears to be impossible in monotonic systems. Many crucial steps in practical reasoning certainly require nonmonotonic behaviors in inference procedures. But, I argue that it is highly misleading to describe these nonmonotonic behaviors in terms of nonmonotonic inference relations `. Rather, inference procedures should use metatheoretic operators, with conventionally monotonic inference relations, to support nonmonotonic behavior. 1 Defeasible Reasoning Requires Nonmono-