Argumentum Ad Alia: argument structure of arguing about what others have said

Expertise, authority, and testimony refer to aspects of one of the most important elements of communication and cognition. Argumentation theory recognises various forms of what we call the argumentum ad alia pattern, in which speakers appeal to what others have said, including Position to Know scheme, Witness Testimony scheme, Expert Opinion scheme and the classical ad verecundiam. In this paper we show that ad alia involves more than merely an inferential step from what others (a person in position to know, a witness, an expert) have said, and that studying this type of argumentation requires going beyond the argument structure of propositional contents to account for different speech activities. We also demonstrate that using the words of others, attacking what has been said, and reasoning from how others reason constitute a rich repertoire of communication strategies which we need to model in order to be able to analyse them manually and to process them computationally.