Limitations on automated legal reasoning

August 24, 2006

Law has appeared promising as a domain for expert systems development to some non-lawyers in the expert system field, who have seen law as a pre-existing set of rules ready for representation in an expert system. As Michie and Johnston put it:

“there are those who are sure that lawyers, for instance, specifically solicitors, could be replaced by computers and that an expert system, albeit a very big one, could actually do the job better. It is true that finding chains of consequences in laws, and finding where laws contradict each other, are ideal tasks for computers and are often done poorly by humans at the moment.”

However, some authorities on expert systems such as such as Waterman have ahown a more sophisticated appreciation of the special problems presented by law as a domain for expert systems:

“… this characteristic of the [legal] domain, having rules that already exist, has led to trouble. The problem this creates is the naive notion (for some) that because a body of rules and regulations exists, all one has to do is to translate them into executable code to create a program for performing complex legal reasoning.

Waterman et al, in the jargon of expert systems research, suggest certain characterisitics of law that make it a more than trivial task to develop legal expert systems:

  1. Its rules are more complex than in most domains, and are “expressed in lengthy natural language passages filled with jargon.”
  2. It is `semi-formalised’, in that although it has a large body of rules, “these rules are often contradictory, incomplete and even deliberately ambiguous”.
  3. Its concepts are `open-textured’, in that it is difficult to state necessary and sufficient conditions for when a legal predicate will apply to a fact situation in a way that a program can apply.
  4. Reduction of complex terms to `more fundamental or primitive concepts’ does not seem possible, with definitions of a complex term often being in terms of equal complexity. Legal reasoning must, therefore, often take place with rather complex predicates. There is no generally applicable deep model of the legal domain, in contrast with other domains.
  5. Commonsense reasoning based on broad `world knowledge’ is often required, making it difficult to maintain a narrow problem domain.
  6. These factors have contributed to “the development of a body of informal knowledge, practices or strategies concerned with how to access and reason with the formal rules.”
  7. `Legal experts use many different kinds of reasoning processes’, ranging from if-then rules to analogical reasoning.

The nature of the `laws’ and `rules’ involved in legal reasoning is one factor causing these difficulties.  It is stresses that medical, chemical and geological expert systems are rooted, ultimately, in the empirically-based, causal, descriptive laws of the natural sciences, whereas legal reasoning involves the manipulation of the prescriptive laws of the legal order, discoverable, in the main, not from uniformities or patterns in the external world but through scrutiny of the formal sources of the law.


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