Common reasons for ontology inconsistency

Summary

Following on from the previous Ontogenesis article “(I can’t get no) satisfiability” (http://ontogenesis.knowledgeblog.org/1329), this post explores common reasons for the inconsistency of an ontology. Inconsistency is a severe error which implies that none of the classes in the ontology can have instances (OWL individuals), and (under standard semantics) no useful knowledge can be inferred from the ontology.

Introduction

In the previous Ontogenesis article “(I can’t get no) satisfiability” (http://ontogenesis.knowledgeblog.org/1329), the authors discussed the notions of “unsatisfiability”, “incoherence”, and “inconsistency”. We recall that a class is “unsatisfiable” if there is a contradiction in the ontology that implies that the class cannot have any instances (OWL individuals); an ontology is “incoherent” if it contains at least one unsatisfiable class. If the ontology is “inconsistent” it is impossible to interpret the axioms in the ontology such that there is at least one class which has an instance; we say that “every class is interpreted as the empty set”.

While incoherent OWL ontologies can be (and are) published and used in applications, inconsistency is generally regarded as a severe error: most OWL reasoners cannot infer any useful information from an inconsistent ontology. When faced with an inconsistent ontology, they simply report that the ontology is inconsistent and then abort the classification process, as shown in the Protégé screenshot below. Thus, when building an OWL ontology, inconsistency (and some of the typical patterns that often lead to inconsistency) needs to be avoided.

In what follows, we will outline and explain common reasons for the inconsistency of an OWL ontology which we separate into errors caused by axioms on the class level (TBox), on the instance level (ABox), and by a combination of class- and instance-related axioms. Note that the examples are simplified versions which represent, in as few axioms as possible, the effects multiple axioms in combination can have on an ontology.

Instantiating an unsatisfiable class (TBox + ABox)

Instantiating an unsatisfiable class is commonly regarded as the most typical cause of inconsistency. The pattern is fairly simple – we assign the type of an unsatisfiable class to an individual:

Individual: Dora
  Types: MadCow

where MadCow is an unsatisfiable class. The actual reason for the unsatisfiability does not matter; the contradiction here is caused by the fact that we require a class that cannot have any instances (MadCow) to have an instance named Dora. Clearly, there is no ontology in which the individual Dora can fulfil this requirement; we say that the ontology has no model. Therefore, the ontology is inconsistent. This example shows that, while incoherence is not a severe error as such, it can quickly lead to inconsistency, and should therefore be avoided.

Instantiating disjoint classes (TBox + ABox)

Another fairly straightforward cause of inconsistency is the instantiation of two classes which were asserted to be disjoint:

Individual: Dora
  Types: Vegetarian, Carnivore
  DisjointClasses: Vegetarian, Carnivore

What we state here is that the individual Dora is an instance of both the class Vegetarian and the class Carnivore. However, we also say that Vegetarian and Carnivore are disjoint classes, which means that no individual can be both a Vegetarian and a Carnivore. Again, there is no interpretation of the ontology in which the individual Dora can fulfil both requirements; therefore, the ontology has no models and we call it inconsistent.

Conflicting assertions (ABox)

This error pattern is very similar to the previous one, but all assertions now happen in the ABox, that is, on the instance level of the ontology:

Individual: Dora
  Types: Vegetarian, not Vegetarian

Here, the contradiction is quite obvious: we require the individual Dora to be a member of the class Vegetarian and at the same time to not be a member of Vegetarian.

Conflicting axioms with nominals (all TBox)

Nominals (oneOf in OWL lingo) allow the use of individuals in TBox statements about classes; this merging of individuals and classes can lead to inconsistency. The following example, based on an example in (10.1145/1060745.1060837), is slightly more complex than the previous ones:

Class: MyFavouriteCow
  EquivalentTo: {Dora}
Class: AllMyCows
  EquivalentTo: {Dora, Daisy, Patty}
  DisjointClasses: MyFavouriteCow, AllMyCows

The first axiom in this example requires that every instance in the class MyFavouriteCow must be equivalent to the individual Dora. In a similar way, the second axiom states that any instance of AllMyCows must be one of the individuals Dora, Daisy, or Patty. However, we then go on to say that MyFavouriteCow and AllMyCows are disjoint; that is, no member of the class  MyFavouriteCow can be a member of AllMyCows. Since we already stated that Dora is a member of both MyFavouriteCow and AllMyCows, the final disjointness axiom causes a contradiction which means there cannot be any interpretation of the axioms that fulfils all three requirements. Therefore, the ontology is inconsistent.

No instantiation possible (all TBox)

The following examples demonstrates an error which may not occur in a single axiom as it is shown here (simply because it is unlikely that a user would write down a statement which is obviously conflicted), but could be the result of several axioms which, when taken together, have the same effect as the axiom below. It is also non-trivial to express the axiom in Manchester syntax (the OWL syntax chosen for these examples) since it contains a General Concept Inclusion (GCI)(http://ontogenesis.knowledgeblog.org/1288), so we will bend the syntax slightly to illustrate the point.

Vegetarian or not Vegetarian
  SubClassOf: Cow and not Cow

Let’s unravel this axiom. First, in order for any individual satisfy the left-hand side of the axiom, it has to be either a member of Vegetarian or not a member of Vegetarian. Clearly, since either something is a member of a class or it is not (there are no values “in between”), the statement holds for all individuals in the ontology. The right-hand side (or, second line) of the axiom then requires all individuals to be a member of the class Cow and not Cow at the same time; again, this falls into the same category as the examples above, which means that no individual can meet this requirement. Due to this contradiction, there is no way to interpret the axiom to satisfy it, which renders the ontology inconsistent.

Conclusion

In this post, we have discussed some of the most common reasons for inconsistency of an OWL ontology by showing – simplified – examples of the error patterns. While some of these – such as instantiation of an unsatisfiable class – can be identified fairly easily, others – such as conflicting axioms involving nominals – can be more subtle.

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