This might be a stupid question but it has been troubling me for quite some while.
I know that OWL has three variants (Full, Lite and DL) and that the newer OWL2 has three profiles (EL, QL and RL).
Now my questions:
- if a semantic reasoner supports OWL2, does that mean that is supports OWL Full too?
- if a reasoner supports OWL2 EL, does it support OWL DL then or is it simply a reasoner that is only focussed on the OWL2 EL profile?
- continuation of the above question, if it supports OWL2 EL, can such a reasoner be used to reason on normal ontologies?
Thx for your time
There is no reasoner that supports OWL 2 Full or OWL Full because they are undecidable - i.e., no reasoner can be complete on these languages.
The OWL and OWL 2 profiles overlap to a point, but do not match exactly - OWL 2 DL includes a few more constructs than OWL DL, and there are features like keys which were not defined at all in OWL.
A reasoner that supports OWL 2 EL can be used on an ontology of any expressivity, but it will not return inferences that are only valid within OWL 2 DL - which means you'll still get correct answers, but they will be incomplete wrt the answers you would get from an OWL 2 DL capable reasoner.
Reasoners that support a less expressive profile can be faster than reasoners that support a more expressive profile, since the underlying worst case complexity of reasoning is lower - i.e., for OWL 2 EL there are polynomial algorithms to compute inferences. That is not true for OWL 2 DL. This does not mean that any OWL 2 DL ontology will take more time to reason than any OWL 2 EL ontology, only that the simpler ontology is more predictable in terms of its requirements.
Related
I have a background in frame-based ontologies, in which classes represent concepts and there's no restriction against assertion of class:class relationships.
I am now working with an OWL2 ontology and trying to determine the best/recommended way to represent "canonical part-of" relationships - conceptually, these are relationships that are true, by definition, of the things represented by each class (i.e., all instances). The part-of relationship is transitive, and I want to take advantage of that so that I'd be able to query the ontology for "all parts of (a canonical) X".
For example, I might want to represent:
"engine" is a part of "car", and
"piston" is a part of "engine"
and then query transitively, using SPARQL, for parts of cars, getting back both engine and piston. Note that I want to be able to represent individual cars later (and be able to deduce their parts by reference to their rdf:type), and of course I want to be able to represent sub-classes of cars as well, so I can't model the above-described classes as individuals - they must be classes.
It seems that I have 3 options using OWL, none ideal. Is one of these recommended (or are any discouraged), and am I missing any?
OWL restriction axioms:
rdfs:subClassOf(engine, someValuesFrom(partOf, car))
rdfs:subClassOf(piston, someValuesFrom(partOf, engine))
The major drawback of the above is that there's no way in SPARQL to query transitively over the partOf relationship, since it's embedded in an OWL restriction. I would need some kind of generalized recursion feature in SPARQL - or I would need the following rule, which is not part of any standard OWL profile as far as I can tell:
antecedent (body):
subClassOf(B, (P some A) ^
subClassOf(C, (P some B) ^
transitiveProperty(P)
consequent (head):
subClassOf(C, (P some A))
OWL2 punning: I could effectively represent the partOf relationships on canonical instances of the classes, by asserting the object-property directly on the classes. I'm not sure that that'd work transparently with SPARQL though, since the partOf relationships would be asserted on instances (via punning) and any subClassOf relationships would be asserted on classes. So if I had, for example, a subclass six_cylinder_engine, the following SPARQL snippet would not bind six_cylinder_engine:
?part (rdfs:subClassOf*/partOf*)+ car
Annotation property: I could assert partOf as an annotation property on the classes, with values that are also classes. I think that would work (minus transitivity, but I could recover that easily enough with SPARQL paths as in the query above), but it seems like an abuse of the intended use of annotation properties.
I think you have performed a good analysis of the problem and the advantages/disadvantages of different approaches. I don't know if any one is discouraged or encouraged. IMHO this problem has not received sufficient attention, and is a bigger problem in some domains than others (I work in bio-ontologies which frequently use partonomies, and hence this is very important).
For 1, your rule is valid and justified by OWL semantics. There are other ways to implement this using OWL reasoners, as well as RDF-level reasoners. For example, using the ROBOT command line wrapper to the OWLAPI, you can run the reason command using an Expression Materializing Reasoner. E.g
robot reason --exclude-tautologies true --include-indirect true -r emr -i engine.owl -o engine-reasoned.owl
This will give you an axiom piston subClassOf partOf some car that can be queried using a non-transitive SPARQL query.
The --exclude-tautologies removes inferences to owl:Thing, and --include-indirect will include transitive inferences.
For your option 2, you have to be careful in that you may introduce incorrect inferences. For example, assume there are some engines without pistons, i.e. engine SubClassOf inverse(part_of) some piston does not hold. However, in your punned shadow world, this would be entailed. This may or may not be a problem depending on your use case.
A variant of your 2 is to introduce different mapping rules for layering OWL T-Tboxes onto RDF, such as described in my OWLStar proposal. With this proposal, existentials would be mapped to direct triples, but there is another mechanism (e.g. reification) to indicate the intended quantification. This allows writing rules that are both safe (no undesired inferences) and complete (for anything expressible in OWL-RL). Here there is no punning (under the alternative RDF to OWL interpretation). You can also use the exact same transitive SPARQL query you wrote to get the desired results.
I am defining an ontology that contains several properties that share range/domain restrictions.
ns:synonym a owl:ObjectProperty ;
rdfs:domain ontolex:LexicalEntry ;
rdfs:range ontolex:LexicalEntry .
ns:antoym a owl:ObjectProperty ;
rdfs:domain ontolex:LexicalEntry ;
rdfs:range ontolex:LexicalEntry .
ns:meronym a owl:ObjectProperty ;
rdfs:domain ontolex:LexicalEntry ;
rdfs:range ontolex:LexicalEntry .
...
I would like to define a subclass of owl:ObjectProperty that will contain all these individual properties.
dbnary:NymProperty a rdfs:Class ;
rdfs:subClassOf owl:ObjectProperty;
<<where individuals has range XXX and domain YYY>> .
ns:synonym a dbnary:NymProperty.
ns:antoym a dbnary:NymProperty.
ns:meronym a dbnary:NymProperty.
...
Is it possible to do this in OWL ? What should I have instead of <<where individuals has range XXX and domain YYY>> ?
What is the impact (necessary reasoning, etc.) on the users of the ontology ?
I need this class of property to be defined as I would like to use these property individuals as range of another relation.
Is it possible to do this in OWL ?
No, but yes.
No because...
When someone asks if something can be expressed in OWL, they usually ask if it is allowed in an OWL Ontology, which is formally defined in the OWL 2 Web Ontology Language Structural Specification and Functional-Style Syntax. According to this specification, it is not possible to define a subclass of owl:ObjectProperty. Most tools that are described as "OWL tools", such as OWL editors (e.g., Protégé) or OWL APIs (e.g., the OWL API) or OWL reasoners (e.g., HermiT) are implementing this specification, which constrains what is allowed in an OWL ontology. In particular, not all RDF graphs can be processed correctly or completely by these tools.
Yes because...
OWL, in its broader definition, comprises multiple standards, including the OWL 2 Web Ontology Language RDF-Based Semantics that defines the notion of an OWL 2 Full ontology, which is a synonym of RDF Graph but with the intention of interpreting it according to the RDF-based semantics of OWL. The RDF-based semantics gives a meaning to any RDF graph, including those that would not be valid representations of OWL Ontologies. According to this formal semantics, it is possible to have subclasses of anything, including owl:ObjectProperty, which would match your requirements. However, the downside is that the RDF-based semantics is an undecidable logic. So the tools that are able to reason with OWL Full ontologies are incomplete. Some triplestore implement a partial axiomatisation of OWL RDF-based semantics, but some valid entailment cannot be derived with these tools.
Note that many existing ontologies do not match the requirements for being standard OWL 2 ontologies. Yet, they are simple enough to be used in many applications. Tools that implement the RDFS semantics of the RDF specification can be seen as simple, incomplete implementations of the OWL 2 RDF-based semantics. Some tools implement a superset of RDFS that can deal with more OWL constructs, but they are still incomplete.
With that said, it is generally ill-advised to use the owl: vocabulary in the definition of an RDF vocabulary, because the logical and practical consequences are hard to foresee. So use this kind of modelling responsibly.
I know that reasoning doesn't work in general when combining SWRL and OWL2 because the combination is not decidable. However, it is decidable when only certain things/rules are used. So assuming I only want to use the SWRL math built-ins, is decibability given or not?
In the W3C explanation of OWL 2 QL it is mentioned that
In particular, this profile (i.e OWL 2 QL) contains the intersection of RDFS and OWL
2 DL.
This sentence is confusing me and I was wondering if somebody can clarify it. Does this mean that OWL 2 QL is a subset of RDFS? Or does this mean that it is a more restricted version of RDFS (and therefore more axioms to express these restrictions).
Assume that I have a KB that has only the following axioms used in it: rdfs:domain, rdfs:range, rdfs:subclassOF, rdf:type, owl:ObjectProperty and owl:DatatypeProperty. What can I say about the language of this KB? can I say it is expressed in RDFS, OWL 2 QL or both?
Is that the case that if I see a resource which is both an instance of something (rdf:type), and a class I would say it is RDFS, and if there is no resource which is both an instance and a class I would say it is also in OWL 2 QL?
If I say, “this box contains the intersection of my t-shirts and green things”, it doesn't mean the box contains only green t-shirts. All my green t-shirts are in the box, but it may contain also blue t-shirts or green socks or whatever else.
In particular, this profile (i.e OWL 2 QL) contains the intersection of RDFS and OWL 2 DL.
The sentence just says: Any KB that meets all the requirements of RDFS and OWL 2 DL happens to also be in OWL 2 QL. But as far as that sentence goes, OWL 2 QL may also contain all sorts of other things that are neither RDFS or OWL 2 DL.
(I am also not sure that the sentence is strictly true. For example, RDFS allows anonymous individuals a.k.a. blank nodes. OWL 2 QL doesn't.)
Your example KB uses OWL constructs, so it is clearly not RDFS. The strict separation between classes and individuals is something from OWL 1 DL, and no longer present in OWL 2 DL.
The terms you mention (rdfs:domain, rdf:type, owl:DatatypeProperty etc.) are not strictly speaking axioms. They are part of the RDF vocabulary that is used to represent OWL axioms if the ontology is represented as an RDF graphs. But the corresponding axioms are DataPropertyDomain, ClassAssertion and DataProperty. When one gets serious about OWL reasoning and the various OWL profiles, it is better to forget about RDF triples for a moment, and to think in terms of actual OWL axioms. For example, the grammar for OWL 2 QL defines exactly what is and is not allowed, but uses the language of OWL axioms and not the language of RDF triples.
What are the OWL ontology language boundaries? Like:
Can I use a class with different parents? (Multiple inheritance) Protege doesn't allow this.
What characters I can or cannot use? e.g. Cannot use '#' or '^' in Protege. Why?
Case-sensitive classes? e.g. class A and a are two different classes?
What else?
The boundaries of OWL are determined by the boundaries of logic of the respective OWL dialect. This is the taxonomy of the OWL2 dialects:
-First Order Logic
--SWRL/RIF
---OWL DL
----OWL EL, RL, QL
-----Concept Hierarchies
--OWL Full
---OWL DL
----OWL EL, RL, QL
-----Concept Hierarchies
---RDFS
-----Concept Hierarchies
You can find more about these dialects here.
The most used dialect is OWL-DL, as it offers a good balance between expressiveness and decidability. There is a classification system for Description Logic to determine expressiveness:
"AL" allows: Atomic negation; Concept intersection; Universal restrictions; Limited existential quantification
"FL" allows:Concept intersection; Universal restrictions; Limited existential quantification; Role restriction
"EL" allows: Concept intersection; Existential restrictions
Then there are the following extensions:
"F" - Functional properties, a special case of uniqueness quantification.
"E" - Full existential qualification
"U" - Concept union.
"C" - Complex concept negation.
"H" - Role hierarchy (subproperties - rdfs:subPropertyOf).
"R" - Limited complex role inclusion axioms; reflexivity and irreflexivity; role disjointness.
"O" - Nominals. (Enumerated classes of object value restrictions - owl:oneOf, owl:hasValue).
"I" - Inverse properties.
"N" - Cardinality restrictions (owl:cardinality, owl:maxCardinality), a special case of counting quantification
"Q" - Qualified cardinality restrictions
"D" - Use of datatype properties, data values or data types.
According to this classification the expressiveness of OWL2-DL is (SHROIQ(D)), where "S" stands for An abbreviation for "ALC" with transitive roles. (Note: there is a terminological difference between DL and OWL, for example OWL specification uses "properties", while DL uses "roles").
So, the short answer to you question is: the boundaries of OWL2-DL are (SHROIQ(D)).
Can I use a class with different parents? (Multiple inheritance)
Protege doesn't allow this
You should be careful when trying to apply metaphors from other modelling paradigms. Strictly speaking "Parents" and "inheritance" are not applicable in OWL. We can say that there is something like sharing of properties but its direction - unlike in the Object Oriented paradigm - is upwards, not downwords. OWL uses "classes" but you should think of them as sets, not as "classes" from OO. Being sets, a class can be as sub-class of different classes and Protégé allows this. In fact it is used quite often. "Boar" is a subclass of both "Bear" and "Male", just as "Bull" would be a subclass of both "Cattle" and "Male". We can always find a set of properties to create a new class. All examples so far would be of course subclasses of "Mammal"and then of "Animal", but they can be also subclasses of e.g. "Two-eyed agents", a class, which can have subclasses that are not animals, for example "two-eyed robots".
What characters I can or cannot use
OWL has different serialisations such OWL/XML, Turtle etc. Each has it's own syntax.
As you asked for useful resources, one such would be of course the OWL primer. I would also recommend this free course.