Argument from Appearance: a New Argumentation Scheme

It is shown how two defeasible forms of argument, argument from appearance and abductive reasoning, are central tools of artificial intelligence for the analysis and evaluation of legal evidence. Defeasible argumentation schemes representing these forms of argument are presented, and applied to examples of the kind of reasoning used to draw a conclusion by inference from observational data. A common example from the Greek philosopher Carneades, the ancient case of the snake and the rope, is used to show how inferences from an appearance to a conclusion about the contents of that appearance are fallible, but can be provisionally acceptable. It is argued that the lessons of this example have not been fully taken advantage of in modern theories of reasoning, and that the best way to come to apply them is to use argumentation tools like argumentation schemes and argument diagramming. Forms of reasoning in which an agent infers a conclusion based on observed data (facts) she has personally seen are extremely important for many reasons. First, they are basic to discovery tools being developed in artificial intelligence (Santos, 2004). Second, they are very important in argumentation theory, where they provide a focus for skeptical attacks, and give rise to controversies about whether such reasoning is inductive or not, and how it can be analyzed. Third, they are very important in scientific method and hypothesis formation, since scientific reasoning of this sort is supposed to be based on observation of data. Verifiability and falsifiability of scientific reasoning is also based on how this form of inference is to be evaluated, and in particular, how it is to be defeated by new data that may indicate revision ∗I would like to thank the Social Sciences and Humanities Council of Canada for a research grant that supported the work in this paper, and to thank Henry Prakken and Chris Reed for discussions that led me to write the paper and strongly influenced my way of thinking about the subject.