Ontology Engineering. Elisa F. Kendall

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      During the reasoning process, the reasoner looks for additional information that it can infer and checks to see if what it believes is consistent. Additionally, since it is generally applying rules of inference, it also checks to make sure it is not in an infinite loop. When some kind of logical inconsistency is uncovered, then the reasoner must determine, from a given invalid statement, whether or not others are also invalid. The process associated with tracking the threads that support determining which statements are invalid is called truth maintenance. Understanding the impact of how truth maintenance is handled is extremely important when evaluating the appropriateness of a particular inference engine for a given task.

      If all new information asserted in a knowledge base is monotonic, then all prior conclusions will, by definition, remain valid. Complications can arise, however, if new information negates a prior statement. “Non-monotonic” logical systems are logics in which the introduction of new axioms can invalidate old theorems (McDermott and Doyle, 1980). What is important to understand when selecting an inference engine is whether or not you need to be able to invalidate previous assertions, and if so, how the conflict detection and resolution is handled. Some questions to consider include the following.

      • What happens if conclusive information to prove the assumption is not available?

      • The assumption cannot be proven?

      • The assumption is not provable using certain methods?

      • The assumption is not provable in a fixed amount of time?

      The answers to these questions can result in different approaches to negation and differing interpretations by non-monotonic reasoners. Solutions include chronological and “intelligent” backtracking algorithms, heuristics, circumscription algorithms, justification or assumption-based retraction, depending on the reasoner and methods used for truth maintenance.

      Two of the most common reasoning methods are forward and backward chaining. Both leverage “if-then” rules, for example, “If it is raining, then the ground is wet.” In the forward chaining process, the reasoner attempts to match the “if” portion (or antecedent) of the rule and when a match is found, it asserts the “then” portion (or the consequent) of the rule. Thus, if the reasoner has found the statement “it is raining” in the knowledge base, it can apply the rule above to deduce that “The ground is wet.” Forward chaining is viewed as data driven and can be used to draw all of the conclusions one can deduce from an initial state and a set of inference rules if a reasoner executes all of the rules whose antecedents are matched in the knowledge base.

      Backward chaining works in the other direction. It is often viewed as goal directed. Suppose that the goal is to determine whether or not the ground is wet. A backward chaining approach would look to see if the statement, “the ground is wet,” matches any of the consequents of the rules, and if so, determine if the antecedent is in the knowledge base currently, or if there is a way to deduce the antecedent of the rule. Thus, if a backward reasoner was trying to determine if the ground was wet and it had the rule above, it would look to see if it had been told that it is raining or if it could infer (using other rules) that it is raining.

      Another type of reasoning, called tableau (sometimes tableaux) reasoning, is based on a technique that checks for satisfiability of a finite set of formulas. The semantic tableaux was introduced in 1950s for classical logic and was adopted as the reasoning paradigm in description logics starting in the late 1990s. The tableau method is a formal proof procedure that uses a refutation approach—it begins from an opposing point of view. Thus, when the reasoner is trying to prove that something is true, it begins with an assertion that it is false and attempts to establish whether this is satisfiable. In our running example, if it is trying to prove that the ground is wet, it will assert that it is NOT the case that the ground is wet, and then work to determine if there is an inconsistency. While this may be counterintuitive, in that the reasoner proposes the opposite of what it is trying to prove, this method has proven to be very efficient for description logic processing in particular, and most description logic-based systems today use tableau reasoning.

      Yet another family of reasoning, called logic programming, begins with a set of sentences in a particular form. Rules are written as clauses such as H :- B1, … Bn. One reads this as H or the “head” of the rule is true if B1 through Bn are true. B1-Bn is called the body. There are a number of logic programming languages in use today, including Prolog, Answer Set Programming, and Datalog.

1.7 EXPLANATIONS AND PROOF

      When a reasoner draws a particular conclusion, many users and applications want to understand why. Primary motivating factors for requiring support for explanations in the reasoners include interoperability, reuse, trust, and debugging in general. Understanding the provenance of the information (i.e., where it came from and when) and results (e.g., what sources were used to produce the result, what part of the ontology and rules were used) is crucial to analysis. It is essential to know which information sources contributed what to your results, particularly for reconcilliation and understanding when there are multiple sources involved and those sources of information differ. Most large companies have multiple databases, for example, containing customer and account information. In some cases there will be a “master” or “golden” source, with other databases considered either derivative or “not as golden”—meaning, that the data in those source databases is not as reliable. If information comes from outside of an organization, reliability will depend on the publisher and the recency of the content, among other factors.

      Some of the kinds of provenance information that have proven most important for interpreting and using the information inferred by the reasoner include:

      • identifying the information sources that were used (source);

      • understanding how recently they were updated (currency);

      • having an idea regarding how reliable these sources are (authoritativeness); and

      • knowing whether the information was directly available or derived, and if derived, how (method of reasoning).

      The methods used to explain why a reasoner reached a particular conclusion include explanation generation and proof specification. We will provide guidance in some depth on metadata to support provenance, and on explanations in general in the chapters that follow.

      ¹ http://www-ksl.stanford.edu/kst/what-is-an-ontology.html.

      ² Extensible Markup Language (XML), see http://www.w3.org/standards/xml/core.

      ³ The Resource Description Framework (RDF) Vocabulary Description Language (RDF Schema), available at https://www.w3.org/RDF/.

      ⁴ See https://schema.org/ for more information.

      ⁵ Structured Query Language, see https://docs.microsoft.com/en-us/sql/odbc/reference/structured-query-language-sql?view=sql-server-2017.

      ⁶ SPARQL 1.1 Query Language, available at https://www.w3.org/TR/sparql11-overview/.

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