D. WALTON AND C. A. REED ARGUMENTATION SCHEMES AND ENTHYMEMES ABSTRACT. The aim of this investigation is to explore the role of argumentation schemes in enthymeme reconstruction. This aim is pursued by studying selected cases of incomplete arguments in natural language discourse to see what the requirements are for filling in the unstated premises and conclusions in some systematic and useful way. Some of these cases are best handled using deductive tools, while others respond best to an analysis based on defeasible argumentations schemes. The approach is also shown to work reasonably well for weak arguments, a class of arguments that has always been difficult to analyze without the principle of charity producing a straw man. 1. INTRODUCTION In many logic textbooks so-called enthymemes, or arguments with miss- ing (unstated) premises or conclusions, are treated using deductive logic (like syllogistic) to reconstruct the given argument. The ten case studies analyzed and discussed below show why this treatment, while it is useful in some cases, is inadequate to treat the broad range of typical kind of cases of enthymemes in natural language discourse in the best way. It is argued that these argument cases cannot best be reconstructed using only deductive forms of reasoning, or inductive forms of reasoning of the modern kind associated with statistical inference. These cases are shown to require a less strict standard of reasoning that is defeasible in nature. This third kind of argumentation has been much studied in artificial intel- ligence (AI), where it is called plausible reasoning, and is often associated with abduction (Josephson and Josephson 1994). It is shown through these case studies that the kind of structure needed to reconstruct the missing parts of an argument is the argumentation scheme (Hastings 1963; Perel- man and Olbrechts-Tyteca 1969; Kienpointner 1987, 1992; Walton 1996; Garssen 2001). It is shown that the argumentation schemes most useful for analysis of many enthymemes are based on defeasible generalizations of a kind that are that are subject to exceptions. 1 This defeasibilistic view of enthymemes is not as new as it may sound to many readers. It can be shown to be very close to what may have been Aristotle’s original doctrine of the enthymeme, according to the view of some commentators Synthese (2005) 145: 339–370 © Springer 2005