The purpose of this chapter is to advance fallacy theory beyond its current state of development by linking it to the notion of defeasible reasoning. Defeasible reasoning has turned out to be very important for computing, especially in view of the attention paid to modeling argumentation (Bench-Capon and Dunne, 2007) and the use of argumentation schemes (Verheij, 2003) and dialogues (Prakken, 2000; 2006) to study problems of nonmonotonic reasoning. The advent of argumentation frameworks (Dung, 1995) can be shown to provide an elegant way of subsuming much previous work on defeasible reasoning (Bondarenko, Dung, Kowalski and Toni, 1997). Although much has been written on individual fallacies, there is comparatively little on the general theory of fallacy, except for the pragma-dialectical theory (van Eemeren and Grootendorst, 1992) and the pragmatic theory (Walton, 1995). Defeasible reasoning is uncontestably important for helping us to better grasp the notion of fallacy and rethink it as a concept useful for modern logic, but so far the link between the two notions has not been studied.

Many of the most common forms of argument associated with major fallacies, such as argument from expert opinion, ad hominem argument, argument from analogy and argument from correlation to cause, have now been analyzed using the device of argumentation schemes (Walton, Reed and Macagno, 2008). Recent research in computing has embraced the use of argumentation schemes and linked them with key logical notions such as burden of proof that are also related to the study of fallacies (Gordon, Prakken and Walton, 2007). Argumentation schemes have been put forward as a helpful way of characterizing structures of human reasoning, such as argument from expert opinion, that have proved troublesome to view deductively. Attempting to deduce the reasonable examples, by viewing the major premise as a conditional not subject to exceptions (e.g., if X says Y, then Y is true) does not work at all well, as this type of argument is typically defeasible.