Inconsistencies in OWL classes

Question

I am new to semantic technologies. I have created some OWL classes and running pellet reasoner to check for inconsistencies in the classes. This is a snippet of what I have created so far:

<owl:NamedIndividual rdf:about="&object_test;obj_5678">
    <rdf:type rdf:resource="&object_test;WorkPiece"/>
    <xyz:widthOfObject rdf:datatype="&xsd;float">0.1</xyz:widthOfObject> <!--X-->
    <xyz:depthOfObject rdf:datatype="&xsd;float">0.1</xyz:depthOfObject> <!--Y-->
    <xyz:heightOfObject rdf:datatype="&xsd;float">0.2</xyz:heightOfObject> <!--Z-->
</owl:NamedIndividual>

<owl:NamedIndividual rdf:about="&xyz;PQR_WorkPiece_5678">
    <rdf:type rdf:resource="&xyz;PQR"/>
    <xyz:eventOccursAt rdf:resource="&object_test;Transform_5678"/>
    <xyz:startTime rdf:resource="&object_test;timepoint_0"/>
    <xyz:objectActedOn rdf:resource="&object_test;obj_5678"/>
</owl:NamedIndividual>

<owl:NamedIndividual rdf:about="&object_test;Transform_5678">
    <rdf:type rdf:resource="&xyz_paramserver;Transform"/>
    <xyz:quaternion rdf:datatype="&xsd;string">0.0 0.0 1.0 0.0</xyz:quaternion>
    <xyz:translation rdf:datatype="&xsd;string">0.5 0.1 0.5</xyz:translation>
</owl:NamedIndividual>

when I run the pellet reasoner

sync_reasoner_pellet(infer_property_values = True, infer_data_property_values = True, debug=2)

there are errors regarding inconsistencies in classes and following is the explanation

This is the output of `pellet explain`: 
 Axiom: Thing subClassOf Nothing

Explanation(s): 
1)   Region subClassOf Abstract
     hasParticipant range Object
     hasRegionDataValue domain Region
     objectActedOn subPropertyOf preActor
     SemanticMapPerception_WorkPiece_1234 objectActedOn obj_1234
     obj_1234 depthOfObject 0.02f
     depthOfObject subPropertyOf hasDepth
     actor subPropertyOf hasParticipant
     Abstract disjointWith Object
     hasShapeParameter subPropertyOf hasRegionDataValue
     preActor subPropertyOf actor
     hasDepth subPropertyOf hasShapeParameter

I am not sure how to read this output. What is this inconsistency?

Answer 1

This refers to a logical inconsistency, saying for example that a shape is both a square and a circle, which is impossible. Something similar is happening in your ontology.

An explanation for an inconsistency consists of a minimal set of axioms and assertions that all must hold for the ontology to be inconsistent. Because it is a minimal set of axioms and assertions , if you can remove any 1 of these axioms or assertions from the ontology, the ontology will be consistent (assuming you have only the 1 explanation for the inconsistency - it is possible to have multiple to explanations for an inconsistency).

From what you provided, you are creating some individuals based on existing ontologies. However, the individuals you provided, are not referred to in the explanation. Hence, they are not the cause of the inconsistency (again, assuming you got only the 1 explanation above).

For more clarity, in the explanation, I have indicated axioms and assertions:

 Region subClassOf Abstract --> axiom
 hasParticipant range Object --> axiom
 hasRegionDataValue domain Region --> axiom
 objectActedOn subPropertyOf preActor --> axiom
 SemanticMapPerception_WorkPiece_1234 objectActedOn obj_1234 --> assertion
 obj_1234 depthOfObject 0.02f --> assertion
 depthOfObject subPropertyOf hasDepth --> axiom
 actor subPropertyOf hasParticipant --> axiom
 Abstract disjointWith Object --> axiom
 hasShapeParameter subPropertyOf hasRegionDataValue --> axiom
 preActor subPropertyOf actor --> 
 hasDepth subPropertyOf hasShapeParameter --> axiom

To figure this out I suggest:

1. Based on the explanation, the problematic individual seems to be obj_1234 or SemanticMapPerception_WorkPiece_1234. I suggest removing these, at least temporarily, and re-run the reasoner. If this is the only explanation, then your ontology should be now consistent. This means how you made assertions about the individuals is incorrect. Hopefully you can find documentation on how to use the ontology or you can contact the creators of the ontology.

2. If you have multiple explanations, try to remove all assertions (at least temporary), and re-run the reasoner. If the ontology is still inconsistent, it means there is a problem with the axioms of the ontology, which you will need to take up with the creators of the ontology.