An Automated System for Argument Invention in Law Using Argumentation and Heuristic Search Procedures*

A heuristic search procedure for inventing legal arguments is built on two tools already widely in use in argumentation. Argumentation schemes are forms of argument representing premise-conclusion and inference structures of common types of arguments. Schemes especially useful in law represent defeasible arguments, like argument from expert opinion. Argument diagramming is a visualization tool used to display a chain of connected arguments linked together. One such tool, Araucaria, available free at http://araucaria.computing.dundee.ac.uk/, helps a user display an argument on the computer screen as an inverted tree structure with an ultimate conclusion as the root of the tree. These argumentation tools are applicable to analyzing a mass of evidence in a case at trial, in a manner already known in law using heuristic methods (Schum 1994) and Wigmore diagrams (Wigmore 1931). In this paper it is shown how they can be automated and applied to the task of inventing legal arguments. One important application is to proof construction in trial preparation (Palmer 2003). New tools have recently been developed for the analysis and evaluation of everyday arguments, notably including argumentation schemes (Walton 1996) and software systems for argument diagramming (Reed and Rowe 2002). They have also been applied to legal argumentation (Friedman 1986; Verheij 2003; Walton 2003), fitting with Wigmore-style diagrams as devices helpful for marshaling evidence, reconstructing argument, visualizing arguments and justifying decisions (Schum 1994; Anderson and Twining 1991). This investigation poses the question of whether these argumentation tools * I thank the Social Sciences and Humanities Research Council of Canada for a research grant and David Godden and Ivan Pupolizio for comments and corrections. 1 When I visited University of Miami Law School in 2004, Professor Terence Anderson asked whether these tools could be applied to invention of arguments. All I could reply at the time An Automated System for Argument Invention 435 © Blackwell Publishing Ltd 2005. could be applied to the invention of legal argumentation. Can the bridge from evaluating a given argument to discovering a new one be crossed? Here it will be shown how argumentation schemes can be used in a heuristic search procedure applied to legal cases using argument diagramming to guide the user in a search for new arguments. Argumentation has shown much promise as a method of identification, analysis and evaluation of arguments that can be applied to problems of reasoning and evidence evaluation in legal cases (Bench-Capon 1997; Verheij 1996; Walton 2002; Bex, Prakken, Reed, and Walton 2003). In the normal kind of case of argument evaluation we are familiar with in logic, the argument is found in a text of discourse, and interpreted through a process of analysis. We identify the premises and conclusion, determine what type of argument it is, and judge by criteria whether the given argument is strong, weak or fallacious. Methods for carrying out these tasks, like argument diagramming and argumentation schemes are in use and currently under development, and are being applied to legal argumentation (Twining 1985; Verhiej 2003; Walton 2003), as well as everyday conversational argumentation (Reed and Norman 2003). Some of these tools, like Wigmore diagrams, have been used to map out the structure of evidence in a trial (Anderson and Twining 1991). Diagramming methods have also been used in artificial intelligence for modeling legal argumentation (Schum 1994; Gordon 1995; Prakken 1997, 2001a; Lodder 1999; Prakken, Reed, and Walton 2003; Verheij 2003; Bex, Prakken, Reed, and Walton 2003). But can the same tools be used to invent arguments? When building a case, for example, in law in preparation for a trial (Palmer 2003), can one use such argumentation tools to search around to discover the best arguments that might be potentially used to support the claim one needs to prove or refute? These are the questions that can only be answered by striking out into new territory. The transition from the task of argument evaluation of a given text of discourse to the task of invention of new arguments seems like a difficult one to make. It represents the ancient gap between logic and rhetoric. Since the time of Plato, there has been a bitter quarrel between these two subjects. Logic sees rhetoric as having no concern for truth, while rhetoric sees logic as abstract and useless for persuasion. It has proved hard to reconcile the two disciplines, even though Aristotle saw them as functionally connected (Hohmann 1990). It will be argued that the key to making the transition is the notion of dialectical relevance, as analyzed in recent work on argumentation (Walton 2002). Relevance is an argumentation concept that is centrally important in law in trial rules, like the Federal Rules of Evidence (Callen 2003). At the same time it is central to argumentation theory generally was that carrying out such a project was possible provided diagramming software could be developed in certain ways. 2 The current version of the rules can be seen at the following web site: http://www.uscourts.gov/rules/newrules4/html (Hohmann 1989; Walton 2004). In this paper it is shown how a diagrammatic method can be used to discover new arguments by searching through a database of facts in a case, using them as premises to seek out a relevant chain of argumentation that aims at an ultimate probandum. The tools currently being used to evaluate arguments are used in a different way to provide a heuristic for inventing new arguments, for example, for proof preparation tasks in a trial. 1. The Breach of Contract Case In this section we take a simple hypothetical case of a typical legal argument in a case and apply a method called argument diagramming to it. Argument diagramming has recently been advocated as a method for analyzing a mass of evidence in a legal case (Friedman 1986; Schum 1994; Lodder 1999). It is also widely used in logic textbooks to identify the premises and conclusions of arguments (Copi 1982; Hurley 2003). The method has also come to be widely used in artificial intelligence as applied to legal argumentation (Gordon 1995; Reed and Norman 2003; Bex, Prakken, Reed, and Walton 2003). The founder of the method of argument diagramming was John H. Wigmore, who used a version of the technique to construct elaborate evidence charts representing the mass of evidence on both sides of a case at trial (Wigmore 1931; Anderson and Twining 1991; Friedman 1986; Schum 1994). To begin our study it will be helpful to see how the argument diagramming technique can be used to represent argumentation typical of the kind found in legal evidence. There is now an extremely helpful software tool available to facilitate argument diagramming called Araucaria (Reed and Rowe 2003). It aids a user when constructing a diagram of the structure of an argument using a simple point-and-click interface, which may be then saved in a portable format called AML, or Argument Markup Language, based on XML (Reed and Rowe 2002). The user inserts the text to be analyzed as a text document into Araucaria. She can then use the cursor to highlight each statement in the text that appears in a left box on the screen. As each statement is highlighted, a circled letter representing it will automatically appear in a right box on the screen. Next the user can draw in arrows from each premise to each conclusion it supports, thus producing an argument diagram connecting all the premises and conclusions in one large diagram that appears in the right box. In a mass of evidence in a trial, the argumentation on each side consists of many premises and conclusions connected to each other by inferences, all connected together in a chain of argumentation. The final conclusion or ultimate probandum is a specially designated proposition that represents the claim to be proved or to be shown doubtful. A set of premises can go together as evidence to support a conclusion in two ways that are commonly distinguished (Freeman 1991). In what is called a linked argument, each premise is dependent on the other(s) to support the conclusion. In what is 436 Douglas Walton © Blackwell Publishing Ltd 2005. called a convergent argument, each premise provides independent evidential support for the conclusion. Let’s begin with the linked argument. To cite the simplest type of example, that of an argument with two premises, if one premise is deleted, the other will fail to offer the same evidential support for the conclusion it did before the deletion. That is the criterion for a linked argument pattern. In a convergent argument, each premise can be seen as a separate argument that stands on its own as evidence supporting the conclusion. Even if one premise is deleted, the other still offers the same evidential support it did before deletion. Thus, in principle, the two patterns are distinct types of support. The following hypothetical case has many of the features of a kind that is very common in legal argumentation. Facts of the Breach of Contract Case Alice signed an agreement to deliver a package to Bob on a certain date. The package contained widgets. The widgets were green. She failed to deliver the package to Bob on that certain date. There was a written contract describing the agreement by Alice to deliver the package. Bob kept two copies of the contract in his desk drawer. The contract contained Alice’s signature, and showed she agreed to deliver the package by the date indicated. Bob sued Alice for breach of contract. By doing so he made a charge against Alice, claiming the ultimate probandum. “Alice is guilty of breach of contract.” Looking over the facts of the case, we need to judge which facts are relevant to Bob’s ultimate probandum. A proposition is relevant in the legal sense if it can used to prove or disprove Bob’s claim. This means that it is relevant if Bob can use it as evidence to support the claim that his ultimate probandum is true, or if Alice could use it to argue that Bob’s argument does not hold up to questioning. Looking over the facts in the case, some can be judged to be relevant in this sense, others not. The facts there were widgets in the package, the fact that the widgets were green, and the fact that Bob kept two copies of the contract in his desk drawer are not relevant. They could turn out to be relevant later, but as things stand, they do not provide evidence that could be used to prove or dispute Bob’s ultimate probandum. These judgments about relevance seem reasonable, but how could we prove them, or at least evaluate them, by some logical method? The evaluation can be carried out by considering whether each proposition has a place in the chain of argumentation used to provide evidence for Bob’s claim. We begin by making a key list representing the propositions at issue in the case. An Automated System for Argument Invention 437 © Blackwell Publishing Ltd 2005. 3 This example, and the analysis of it continued in section 5 below, can also be found in Walton 2005. Key List for the Breach of Contract Case (A) Alice is guilty of breach of contract. (B) There was an offer made by someone for Alice to carry out some action. (C) Alice accepted that offer. (D) Alice failed to carry out the action. (E) Alice signed an agreement to deliver a package to Bob on a certain date. (F) Alice failed to deliver the package to Bob on that date. (G) Bob showed a written contract describing the agreement by Alice to deliver the package. (H) The contract contained Alice’s signature, and showed she agreed to deliver the package by the date indicated. To begin the analysis of the argumentation in the case, we need to see how inferences drawn from the facts make up a chain of argumentation supporting Bob’s ultimate probandum. To prove the existence of a contract in law, you have to begin by proving that there was an offer made by one party and an acceptance of the offer by the other. Bob has to prove these facts by showing that he made an offer to pay something for Alice to deliver the package by a specified date, and that she agreed to deliver the package by that date. Then to prove breach of contract, he has to prove that she failed to deliver the package by that date. This chain of argumentation is represented in the Araucaria diagram below. 438 Douglas Walton © Blackwell Publishing Ltd 2005. (A) Alice is guilty of (B) There was an (C) Alice accepted (D) Alice failed to (F) Alice failed to deliver the package to Bob on that date. carry out the action.