Overview of Representation Technique

In this section, we provide brief overviews about OWL ontologies, the Protégé ontology editor and knowledge acquisition tool, and the ACE View Protégé plugin.

2.1. OWL Ontologies and Protégé

We outline some of the main reasons for providing an ontology in OWL and using the ontology development tool Protégé ([2] and [3]), with particular refer-ence to issues related to ontologies in the legal domain ([4] and [5]).

An ontology explicitly and formally specifies a conceptualisation of the prop-erties of and relations between objects in a given domain ([6, p.10], [7], [2], and [3]). In the legal domain, ontologies have been found useful as well ([4], [5], and [8]). A common vocabulary and framework is provided in an ontology so that researchers can share, test, and modify the conceptualisation. To provide an on-tology, one gives classes of objects along with their properties such as attributes;

one also specifies the relationships among the various objects of the ontology.

Given instances of the classes, we have a knowledge base. One can then apply rules such as production rules to elements of the knowledge base to draw addi-tional inferences from the knowledge base.

We have developed our case ontology in the Ontology Web Language (OWL) using the Protégé ontology editor and knowledge acquisition tool. OWL provi-des a machine readable ontology which can then be processed by Semantic Web applications. While OWL provides a range of flavours, each associated with a degree of logical expressiveness and associated computational properties, for our purposes, OWL Lite has been sufficient. The Protégé ontology editing and knowledge acquisition tool supports systematic development of an ontology

An OWL Ontology for Legal Cases with an Instantiation of Popov v. Hayashi 23

along with structured instantiations; we have used a recently released version Protégé 4.0 Beta which supports ontologies for so-called Web 2.0 in which rich semantic information and rules can be processed over the internet. Protégé enables users to query the knowledge base, test an ontology for consistency, draw inferences, and apply rules to elements of the knowledge base. However, as our focus is on formally modeling a legal case, we primarily present the on-tology; moreover, reasoning about cases presupposes the ontology.

2.2. ACE View

One additional feature of Protégé is that it provides a plugin framework, al-lowing the development of a range of tools. We have used the tool ACE View, which is an ontology and rule editor that uses Attempto Controlled English (ACE) to create, view and edit OWL 2 ontologies and SWRL rulesets². At-tempto Controlled English (ACE) is a controlled natural language, where a controlled language is a formal specification of a subset of English with a large vocabulary and an expressive set of grammatical constructions³. Once having learned how to author expressions (which is relatively easy), expressions in ACE write and read as standard English. As a formal language, ACE texts are computer-processable and unambiguous. Sentences are parsed and translated into Discourse Representation Structures, which provide a syntactic variant of first-order logic; ACE provides for consistency checks and inferencing. ACE View supports bidirectional translation of expressions between ACE and OWL description logic. With ACE View, one can create OWL/SWRL knowledge bas-es, open OWL 2 ontologies and view them as ACE texts, and edit OWL/SWRL knowledge bases. ACE View has a variety of views:

– ACE Snippet Editor: This allows single sentences to be added to or de-leted from an active ACE text (the text which is being translated into OWL).

– ACE Text: The active ACE text as plain text.

– Lexicon: The active lexicon as a table.

– ACE Q & A: Questions and their answers in the active ACE text – ACE Entailments: The entailments derived from the active ACE text.

For example, we can enter each of the following sentences:

1. Every man is a human.

2. Bill is a man.

3. Who is a human?

2 http://attempto.ifi.uzh.ch/aceview/

3 http://attempto.ifi.uzh.ch/site/

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In this example, a man represents the class of entities which are men as op-posed to the existential interpretation found in A man walked in. Unless made explicit, it is not supposed that relations are functional; for example, a case may be functionally defined to have only one citation index, but allow for the same case to have several hearing dates.

The sentences [1]-[3] are syntactically parsed and mapped to correlelated elements in an OWL ontology. The term man and human are classes, where the class human is a subclass of thing and where man is a subclass of human.

“Bill” is introduced as an individual in the ontology. The ontology is graphically represented in Figure 1.

Thing is a is a

human man

Figure 1: Graphic of an Ontology written in ACE View

Applying the reasoner Pellet, we can open the ACE Q & A view, highlight the question Who is a human?, and see the answer “Bill”. We can also view seria-lised versions of the ontology such as this assertion that Bill is a man; note the annotation for the acetext.

<ClassAssertion>

<Annotation annotationURI=”&acetext;acetext”>

<Constant>Bill is a man.</Constant>

</Annotation>

<Class URI=”&TESTTEST01;man”/>

<Individual URI=”&TESTTEST01;Bill”/>

</ClassAssertion>

For our purposes, it is clearer, easier, and more comprehensible to simply present the ontology with the sentences. While it may be that in some circum-stances a picture is worth a thousand words, in our context, the linguistic rep-resentation in the ontology maintains a closer link to the linguistic representa-tion of the case. It is also a more accessible format for legal professionals who may then be involved in developing and maintaining the ontology.

As a controlled language ACE is restricted, yet it is nonetheless expressive.

Object properties are predicates of subjects and are expressed with a transitive verb phrase (or adjectival phrase) that contains a direct object with a proper name or previously introduced individual; for example, in Every man knows John, the transitive verb phrase is knows John predicates of the subject Every man. Data properties use the copula “is”, a genitive construction, and a noun that expresses the property; for example, in Jocasta’s child is Oedipus, the property of having a child ascribed to the individual Jocasta has the value Oe-dipus. The language can also express complex classes using negation,

disjunc-An OWL Ontology for Legal Cases with an Instantiation of Popov v. Hayashi 25

tion, and conjunction; there are quantifiers for noun phrases (e.g. every, no, a).

A disjoint class would be expressed in ACE as No man is a woman. Properties can be transitive, irreflexive, symmetrical, and so on; for example, symmetry of the love relation is expressed as Everybody who is loved by somebody loves him.

If two properties are equivalent, they are superproperties of one another: Ev-erybody who hates somebody despises him and EvEv-erybody who despises some-body hates him. ACE has a vocabulary of approximately 100,000 words.

There are a range of restrictions in ACE View. For example, adjectives in noun phrases (e.g. “tall” in The tall woman is happy) are allowed in ACE, but not in ACE View. There are a range of other grammatical constructions which are not available, but for our current purposes this is not central; for example, as ACE View adheres to OWL, it is not as expressive as SWRL rules. Once one learns the range of grammatical constructions, then the language is easy and expressive. If one wants to introduce complex terms that are not available in ACE, they can be used as basic terms such as precedent_case. We use this in our sample ontology. As the expressiveness of ACE View extends, so too will its utility as a tool for ontological representation. Further details about the capabilities of ACE in general and ACE View in particular are available on the website.