A central topic in the theory of argumentation is argument evaluation. For any particular argument, the question can be asked whether it is good or bad, rational or irrational, valid or invalid, reasonable or unreasonable. Formal logic has addressed the topic of argument evaluation focusing on certain idealized classes of arguments, like those involving the truth-functional connectives. In formal logic, argument evaluation is normally discussed in terms of a formal semantics or of inference rules. The focus of informal logic is on natural language, real-life arguments, for instance as they occur in the media, in scientific debate or in the court room. Discussion of argument evaluation in informal logic typically involves fallacies like the argumentum ad baculum (appeal to force) and argumentum ad verecundiam (inappropriate appeal to authority).
A productive author in the field of informal logic and fallacies is Douglas Walton. The list of books by Walton is impressive: since 1989 he has published 16 titles. Walton's work can roughly be divided into two categories. First there are the books in which he expounds his theoretical framework for the analysis and evaluation of argumentation. Examples are A Pragmatic Theory of Fallacy (1995), Argument Structure: A Pragmatic Theory (1996) and The New Dialectic: Conversational Contexts of Argument (1998). Second there are the books in which a specific type of argumentation or fallacy is addressed. Examples are Appeal to Expert Opinion: Arguments from Authority (1997), Ad Hominem Arguments (1998) and One-Sided Arguments. A Dialectical Analysis Of Bias (1999).
A starting point in Walton's work is that argumentation can only be rightly appreciated in its conversational context. As a consequence, in order to evaluate a particular instance of argumentation as good or bad, it does not suffice to analyze it as a structured series of statements that express a line of reasoning. Similarly relevant for the evaluation of argumentation is the dialogue context in which it occurs. In Walton's theory, amongst others, the dialogue type and goal can determine whether an argument is good or bad.
It turns out that Walton addresses many topics that are also dealt with by researchers in artificial intelligence and law, such as the relation between dialogue and argument evaluation, the defeasibility of arguments, and the specification of particular kinds of arguments. Walton aims mainly at the informal logic community and other readers with a theoretical or practical interest in the analysis and evaluation of actual argumentation. His style is not formal and as such very different from that of the formally oriented work in artificial intelligence and law. As a result, Walton's work can provide a refreshing perspective on a number of familiar themes and inspire future formal work.
In the following, two aspects of Walton's theoretical framework are discussed that play a central role in the three books under review: dialogue types and argumentation schemes. Dialogue types and their relation to argument evaluation are the central topic of The New Dialectic: Conversational Contexts of Argument. Walton applies his theory of dialogue types to the evaluation and analysis of biased arguments in One-Sided Arguments. A Dialectical Analysis Of Bias. Dialogue types are discussed below in section 2. Argumentation schemes are the basic tool in Walton's analysis of personal attack arguments in Ad Hominem Arguments. Section 3 is about argumentation schemes.
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