Legal Reasoning and Argumentation Douglas Walton University of Windsor CRRAR McMaster University October 8, 2012.

Legal Reasoning and Argumentation

Douglas Walton University of WindsorCRRAR

McMaster University October 8, 2012

Artificial intelligence, taking advantage of new argumentation tools, has lent support to Wigmore’s view that there is a science of proof underlying legal reasoning different from deductive logic (Sartor, 2005; Prakken, 2005, 2006). In this talk it is shown how recent advances in argumentation suggest the value of modeling legal reasoning in this new way.

The first section and studies the kind of reasoning that applies rules to cases in law, including the following forms of reasoning: argument from an established rule, argument from a verbal classification, and argument from precedent. By using an example from a Supreme Court case summary, it is shown how all three kinds of reasoning can be combined into a chain of reasoning that represents the structure of the evidence used to support a conclusion to be proved in a case.

The second section discusses argument from analogy.The third section extends the analysis to two forms of reasoning that draw from conclusions from sources, argument from witness testimony and argument from expert opinion. Throughout, it is hinted at how the structure of evidential reasoning exhibited in the other sections can be modeled in the Carneades Argumentation system.The fourth section briefly shows how the notion of proof, including the notions of standard of proof and burden of proof, need to be defined within a procedural context of argumentation that has three main stages.

The term ‘defeasible’ comes from medieval English contract law. It referred to a contract that has a clause in it that could defeat the contract in a case the circumstances of the case fit the clause. This meaning is now broadened to include the notion of a defeasible rule, a rule that is open to exceptions. Hart, in his famous paper ‘The Ascription of Responsibility and Rights’ (1949; 1951), extended the usage of this term even further by writing about defeasible concepts. His most famous example is from The Concept of Law (1961). Consider the rule that no vehicles are allowed in the park. This rule could be defeated by special circumstances, for example during a parade, but it could also be defeated because of the open texture of the concept of a vehicle. Even though a car is classified as a vehicle, and would be excluded from the park, it may be debatable whether other objects like a bicycle or a skateboard also fit into the same classification.

Typical argumentation schemes (a) represent defeasible, plausible arguments that depend on common understanding of the way things can normally be expected to go in a kind of case familiar to speaker and hearer, and (b) depend on warrants that are generalizations that hold only subject to exceptions.

ARGUMENT FROM AN ESTABLISHED RULE (Walton, Reed and Macagno, 2008, 343). Major Premise: If carrying out types of actions including the state of affairs A is the established rule for a, then (unless the case is an exception), a must carry out A. Minor Premise: Carrying out types of actions including state of affairs A is the established rule for a. Conclusion: Therefore a must carry out A.In this form of reasoning, a is an agent that is capable of carrying out goal-directed actions and recognizing the consequences of actions.

This meaning is now broadened to include the notion of a defeasible rule, a rule that is open to exceptions. Hart, in his famous paper ‘The Ascription of Responsibility and Rights’ (1949; 1951), extended the usage of this term even further by writing about defeasible concepts. His most famous example is from The Concept of Law (1961). Consider the rule that no vehicles are allowed in the park. This rule could be defeated by special circumstances, for example during a parade, but it could also be defeated because of the open texture of the concept of a vehicle. Even though a car is classified as a vehicle, and be excluded from the park, it may be debatable whether other objects like a bicycle or a skateboard also fit into the same classification.

Consider the case of the drug-sniffing dog (Brewer, 1996, Weinreb, 2005). Suppose a trained dog sniffs luggage left in a public place and signals to the police that it contains drugs. Should this event be classified as a search according to the Fourth Amendment? If so, the evidence so obtained is not admissible as evidence. The problem is that the concept of a search is defeasible and law cannot define it by means of a set of necessary and sufficient conditions for closed to future revision because new cases may arise.

ARGUMENT FROM VERBAL CLASSIFICATION(Walton, Reed and Macagno, 2008, 319). Here the constant a represents an individual that can be an object of any kind, including an event, a physical object, an animal or a human being. Individual Premise: a has property F. Classification Premise: For all x, if x has property F, then x can be classified as having property G. Conclusion: a has property G.

ARGUMENT FROM PRECEDENT Previous Case Premise: the source case is a previously decided case. Previous Ruling Premise: In the source case, rule R was applied and produced finding F. New Case Premise: the target case is a new case that has not yet been decided. Similarity Premise: the target case is similar to the source case in relevant respects. Conclusion: Rule R should be applied to the target case and produce finding F.

Arguments from ordinary meaning express the principle that if an argument can be interpreted according to the meaning a native speaker of a given language would ascribe to it, the argument should be interpreted in this way, unless there is a reason for a different interpretation.

In this U.S. Supreme Court case (CSX Transportation, Inc. v. Alabama Department of Revenue et al. certiorari to the U.S. U.S. Court of Appeals for the eleventh circuit No. 09-520, decided February 22, 2011) CSX, claimed that the State of Alabama had discriminated against them (http://www.supremecourt.gov/opinions/10pdf/09-520.pdf). The State taxes diesel fuel consumed by railroads but exempts interstate motor and water carriers. CSX claimed that this tax scheme discriminates against railroads in violation of the Railroad Revitalization and Regulatory Reform Act of 1976 which bars discriminatory taxation.

The key question thus becomes whether a tax might be said to “discriminate” against a railroad under subsection (b)(4) where the State has granted exemptions from the tax to other entities (here, the railroad's competitors). Because the statute does not define “discriminates,” the Court again looks to the term's ordinary meaning, which is to fail to treat all persons equally when no reasonable distinction can be found between those favored and those not favored. To charge one group of taxpayers a 2% rate and another group a 4% rate, if the groups are the same in all relevant respects, is to discriminate against the latter. That discrimination continues if the favored group’s rate goes down to 0%, which is all an exemption is.

To say that such a tax does not “discriminate” is to adopt a definition at odds with the word's natural meaning. This Court has repeatedly recognized that tax schemes with exemptions may be discriminatory. See, e.g., Davis v. Michigan Department of Treasury, 489 U. S. 803. And even Department of Revenue of Ore. v. ACF Industries, Inc., 510 U. S. 332, on which the Eleventh Circuit heavily relied in dismissing CSX's suit, made clear that tax exemptions “could be a variant of tax discrimination.” Id., at 343. In addition, the statute's prohibition of discrimination applies regardless whether the favored entities are interstate or local. The distinctions drawn in the statute are not between interstate and local actors, as Alabama suggests, but between railroads and all other actors, whether interstate or local.

The version of the basic scheme for argument from analogy from (Walton, Reed and Macagno, 2008, 315) is presented below. Similarity Premise: Generally, case C1 is similar to case C2. Base Premise: A is true (false) in case C1. Conclusion: A is true (false) in case C2.

An argument fitting this scheme can be evaluated by asking one or more of the following set of critical questions. CQ1: Are there respects in which C1 and C2 are different that would tend to undermine the force of the similarity cited? CQ2: Is A the right conclusion to be drawn in C1? CQ3: Is there some other case C3 that is also similar to C1, but in which some conclusion other than A should be drawn?

The basis for deciding whether one case is a precedent for another in law is that it is based on an underlying argument from analogy (Walton, D., Similarity, Precedent and Argument from Analogy Artificial Intelligence and Law, 18 (3), 2010, 217-246) .But how is a case where a man sued a company because there was a decomposed snail in his beer bottle similar to a case where a man tried to sue because of a defective Buick automobile? My answer: there is something about the common sequence of events that makes the one case similar to the other. First the plaintiff bought some product that he assumed was the normal product he expected, and he thought therefore that the product was reasonably safe to use. Then something in the product turned out to be defective, and when he used the product this defect caused some harm that impacted badly on his health.

Script as the Basis of the SimilarityPremise in Argument from Analogy

Karen Silkwood was a chemical technician who worked for the Kerr-McGee Corporation in their nuclear power plant in Oklahoma. She was active in the union. On November 5, 1974 she discovered that she had been exposed to dangerously high levels of plutonium radiation. High levels of radioactive contamination were found in her apartment. She was sent to Los Alamos Scientific Laboratory, where they documented evidence of radioactive contamination. After her return she arranged to meet a reporter from the New York Times to say that safety and quality controls at Kerr-McGee on the making of the fuel rods had been falsified. On the way to the meeting, she died in a mysterious one-car accident. Silkwood’s apartment was quarantined, and her personal property from it was buried in a nuclear waste site. Judgment Reversed by Silkwood v. Kerr-McGee Corp., 464 U.S. 238, 104 S.Ct. 615, 78 L.Ed.2d 443, 20 ERC 1229, 14 Envtl. L. Rep. 20,077 (U.S.Okla. Jan 11, 1984) (NO. 81-2159)

The other scheme for argument from analogy does not use the notion if similarity. Instead it compares specific respects in which two cases are similar (Gurarini, 2004). Respects Premise: Case C1 is similar to case C2 in a certain respect. Base Premise: A is true (false) in case C1. Conclusion: Support is offered to the claim that A is true (false) in case C2.The second scheme is especially useful for case-based reasoning.

Gerry Spence used an analogy in his famous closing argument. The trial began in March 1979 and took 11 weeks. The jury returned a verdict for $10,505,000, and when Kerr-McGee appealed, punitive damages were settled at $1.38 million (Meyer, 2002, 235). Spence compared the Silkwood case to a familiar kind of example from the history of English common law often used to illustrate strict liability. In this example someone was keeping a lion in a cage on his property when the lion escaped, through no fault of its owner, and attacked some people, harming them. The owner was held responsible for the harm.

Case-Based Reasoning

• CATO (Aleven, 1997) is based on factors representing respects in which a case is similar to or different from another one (critical questions).

• This can happen in case-based reasoning when some factors support the argument while others detract from it.

• In case-based reasoning argument from analogy is a defeasible form of argument in which further evidence can be introduced that can go against or even defeat the argument.

• To weigh the arguments on each side, we have to consider the factors on each side, and determine which factors are more “on-point”, or relevant.

Sequence of Case-Based Reasoning

A rule of law applied to the source case.

Similarity is seen between source and target case.

The same rule of law is applied to the target case.

Rule of law may fit the target case.

Rule of law may be qualified, or a new rule may even be formulated.

Difference between two cases may be found.

Different facts are emphasized as important by a different judge.

New rule of law can be applied to a third case.

Third case becomes a first case in a new sequence.

One of the most important forms of source-based reasoning and law is inference from witness testimony. The argumentation scheme for this form of reasoning is given in (Walton, 2008, 60). It has three premises. Position to Know Premise: Witness W is in a position to know whether A is true or not. Truth Telling Premise: Witness W is telling the truth (as W knows it). Statement Premise: Witness W states that A is true (false). Conclusion: Therefore (defeasibly) A is true (false).

Six Critical Questions Matching the Argument from Witness Testimony Internal Consistency Question: Is what the witness said internally consistent? Factual Consistency Question: Is what the witness said consistent with the known facts of the case (based on evidence apart from what the witness testified to)? Consistency with Other Witnesses Question: Is what the witness said consistent with what other witnesses have (independently) testified to? Trustworthiness Question: Is the witness personally reliable as a source? Plausibility Question: How plausible is the statement A asserted by the witness? Exception if what the witness says is implausible. Bias Question: Is there some kind of bias that can be attributed to the account given by the witness?

The most basic version of the form of argument from expert opinion is modified from the one in (Walton, Reed and Macagno, 2008, 310) as follows. Major Premise: Source E is an expert in field F. Minor Premise: E asserts that proposition A is true (false). Second Minor Premise: A is within F. Conclusion: A is true (false).

Legal Expert Opinion Scheme• Competence Premise (Ordinary Premise): E is an expert in knowledge

domain D.• Statement Premise (Ordinary Premise): E said the sentence S*.• Interpretation Premise (Ordinary Premise): S is a reasonable interpretation

of S*.• Domain Premise (Assumption): S is in D.• Depth of Knowledge Premise (Assumption): The knowledge of E about D is

deep enough to know about S.• Careful Analysis Premise (Assumption 3): E’s testimony S* is based on his

own careful analysis of evidence in this case.• Other Experts Premise (Exception): S is inconsistent with what other

experts in D say.• Trustworthiness Premise (Exception): E is not trustworthy.• Conclusion: S may plausibly be taken to be true.

Expertise Question: How knowledgeable is E as an expert source?Field Question: Is E an expert in the field F that A is in?Opinion Question: What did E assert that implies A?Trustworthiness Question: Is E personally reliable as a source?Consistency Question: Is A consistent with what other experts assert?Backup Evidence Question: Is E’s assertion based on evidence?

Salmon’s Case (Wigmore)

 Salmon sold medicines in London. M’Kensie bought from him some pills for rheumatism; after numerous doses he died. The medical men, on a postmortem examination, affirmed that certain ingredients of these pills had caused his death; and Salmon was indicted for manslaughter. But on the trial he produced many witnesses, who had taken the same kind of pills with much benefit; one witness affirmed that he had taken twenty thousand of them within the past two years, to his great benefit! If these circumstances were true, the inference was inevitable that the pills were not lethal. But was the testimony to this circumstance true?

Araucaria Diagram of Salmon Case

Carneades takes the approach that the way critical questions are modeled depends on the individual argumentation scheme, by distinguishing three kinds of premises. Ordinary premises are just the regular premises of an argumentation scheme that are explicitly given in the scheme itself. But there are two additional kinds of premises not stated in the scheme. Assumptions are to be acceptable unless called into question. Exceptions are modeled as premises that are not assumed to be acceptable and that can defeat an argument as it proceeds. Ordinary premises of an argument, like assumptions, are assumed to be acceptable, but they must be supported by further arguments in order to be judged acceptable.

Proof Standards in Law• There are four main proof standards for factual issues in common law (B.A. Garner,

Black’s Law Dictionary (9th ed.), Thomson Reuters, St Paul, Minn., 1990, 241). • The scintilla of evidence proof standard is met if “even the slightest amount of

relevant evidence exists on an issue” (1464). • The preponderance of evidence standard is met by “evidence that has the most

convincing force, superior evidentiary weight that […] is still sufficient to incline a fair and impartial mind to one side of the issue rather than the other” (1301).

• Clear and convincing evidence is “evidence indicating that the thing to be proved is highly probable or reasonably certain” (636). This standard is supposed to be higher than that of preponderance of the evidence, but not as high as the highest standard in law, that of evidence beyond a reasonable doubt.

• The beyond reasonable doubt standard is used to determine guilt in criminal cases, and is often equated with the presumption that the defendant is innocent.

Standards of Proof (Carneades) Scintilla of Evidence (SE) is met if there is at least one applicable

argument for a claim. Preponderance of the Evidence (PE) is met if SE is satisfied and the

maximum weight assigned to an applicable pro argument (for the claim) is greater than the maximum weight of an applicable con argument (against the claim).

Clear and Convincing Evidence (CCE), is met if PE is satisfied, the maximum weight of applicable pro arguments exceeds some threshold α, and the difference between the maximum weight of the applicable pro arguments and the maximum weight of the applicable con arguments exceeds some threshold β.

Beyond Reasonable Doubt (BRD)is met if CCE is satisfied and the maximum weight of the applicable con arguments is less than some threshold γ.

In law, there is a fundamental distinction between two main types of burden of proof (Prakken and Sartor, 2009). One is the setting of the global burden of proof at the opening stage, called the burden of persuasion. It does not change during the argumentation stage, and is the device used to determine which side has won at the closing stage. The other is the local setting of burden of proof at the argumentation stage, often called the burden of production in law. This burden can shift back and forth as the argumentation proceeds. For example, if one side puts forward a strong argument, the other side must meet the local burden to respond to that argument by criticizing in or presenting a counter-argument, or otherwise the strong argument will hold.

BRD and IBE• It is regarded as very dangerous for a judge to try to define ‘beyond

reasonable doubt’ in a criminal trial, as there is judicial hostility to attempting any precise definition, and there is a very real danger of appeal, because such a precization is not established in precedent.

• There is a vocal acceptance in law of the view that the beyond reasonable doubt standard cannot be quantified by using numbers. That does not mean it cannot be modeled using computational tools.

• Pardo and Allen (2007, 238) argue that it can modeled using IBE: “In criminal cases, rather than inferring the best explanation from the potential ones, fact-finders infer (and should infer) the defendant’s innocence whenever there is a sufficiently plausible explanation of the evidence consistent with innocence (and ought to convict when there is no plausible explanation consistent with innocence, assuming there is a plausible explanation consistent with guilt).

A dialogue is formally defined as an ordered 3-tuple (O, A, C) where O is the opening stage, A is the argumentation stage, and C is the closing stage (Gordon & Walton, 2009, p. 5). Dialogue rules (protocols) define what types of moves are allowed by the parties during the argumentation stage (Walton & Krabbe, 1995). At the opening stage, the participants agree to take part in some type of dialogue that has a collective goal. Each party has an individual goal and the dialogue itself has a collective goal. The initial situation is framed at the opening stage, and the dialogue moves through the opening stage toward the closing stage.

ReferencesGordon, T. F. (2010). The Carneades Argumentation Support System, Dialectics, Dialogue and Argumentation, ed. C. Reed and C. W. Tindale, London: College Publications.Gordon, T. F. and Walton, D. (2009). Proof Burdens and Standards. Argumentation and Artificial Intelligence, ed. I. Rahwan and G. Simari. Berlin: Springer, 239-260. Prakken, H. (2010).On the Nature of Argument Schemes. Dialectics, Dialogue and Argumentation, ed. Chris Reed and Christopher W. Tindale, London: College Publications, 167-185.Reed, C. and Walton, D. (2003). Diagramming, Argumentation Schemes and Critical Questions, Anyone Who Has a View: Theoretical Contributions to the Study of Argumentation, ed. F. H. van Eemeren, J. A. Blair, C. A. Willard and A. Snoek Henkemans. Dordrecht: Kluwer, 195-211.Verheij, B. (2001). Legal Decision Making as Dialectical Theory Construction with Argumentation Schemes, The 8th International Conference on Artificial Intelligence and Law: Proceedings of the Conference, New York Association for Computing Machinery, 225-236. Available at http://www.ai.rug.nl/~verheij/publications.htm.Walton, D. Appeal to Expert Opinion. University Park: Penn State Press, 1997.Walton D. and Gordon, T. F. (2005). Critical Questions in Computational Models of Legal Argument, Argumentation in Artificial Intelligence and Law, IAAIL Workshop Series, ed. Dunne, P. E. and T. J. M. Bench-Capon. Nijmegen: Wolf Legal Publishers, 103-111.Walton, D., Reed, C. and Macagno, F. (2008). Argumentation Schemes, Cambridge: Cambridge University Press.