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Issue #qu-03 ontology subset of rdfs

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Thu, 20 Feb 2003 13:26:16 +0000
Message-Id: <5.1.0.14.0.20030220132438.02fd0f50@localhost>
To: pat hayes <phayes@ai.uwf.edu>, "Qu Yuzhong" <yzqu@seu.edu.cn>
Cc: "rdf-comments" <www-rdf-comments@w3.org>

At 12:36 19/02/2003 -0600, pat hayes wrote:
>> >
>> > >{pat wrote]
>> > >...
>> > >
>> > >>  I would ask for this notion of 'role' to be clarified before giving a
>> > >>  detailed response. The existing RDFS model theory does not recognize
>> > >>  any 'roles'.
>> > >
>> > >Yes, the existing RDFS model theory does not recognize any dual 'roles'.
>> > >
>> > >Based on my understanding, RDFS (RDF and RDF Schema) can be used as
>> > >a meta-language (a language for defining other ontology languages)
>> > >as well as an ontology language.
>> >
>> > That is not exactly my understanding, although we may be simply using
>> > terminology slightly differently.
>> >
>> > RDF is a language for making simple assertions about things (which
>> > can be anything, in principle). The primary intended purpose of
>> > RDF(S) is to provide for semantic markup of web pages to express
>> > content in a machine-usable form. It is essentially a small subset of
>> > a first-order logic. It can be, and is intended to be, used as a
>> > foundation for other, richer, languages which are extensions of RDF
>> > (and include RDF as sublanguages) but it is not accurate to describe
>> > this as using RDF to *define* other languages, since these other
>> > languages (including RDFS and OWL) in fact cannot be defined in RDF:
>> > they require extra semantic conditions which need to be stated
>> > explicitly in a specification document.
>>
>>Understood. Thanks!
>>As you pointed, the RDFS can be, is intended to be, used as a foundation 
>>for other, richer, languages which are extensions of RDF.
>>My point is that the another role of the RDFS is as an ontology language, 
>>but the RDFS as an ontology language is not the full RDFS, it should be 
>>defined by some constraints.
>
>OK, so I think now that I follow your point. Is this a fair summary?:
>
>There is a class of languages called 'ontology languages'.  Considered as 
>an ontology language, RDFS provides too much expressive power, and the 
>extra power is inappropriate for an ontology language. Therefore, it would 
>be desirable if a subset of RDFS could be identified which conforms to the 
>requirements for an ontology language (by imposing constraints on the 
>ability to define classes of classes, classes of properties and properties 
>of classes, as outlined later in your message.)
>
>If so, I will suggest that Brian give this an issue number for 
>consideration by the WG.

Qu,

I have recorded this comment at

   http://www.w3.org/2001/sw/RDFCore/20030123-issues/#qu-03

The WG will consider this and respond in due course.

Once again, thank you for time and effort commenting on the RDFCore WD's.

Brian



>> > >The basic difference between them can be figured out as follows:
>> > >
>> > >1. RDFS as an ontology language.
>> > >
>> > >It can be defined by following constraints (exclusion approach):
>> >
>> > BUt it is not appropriate for RDF to incorporate such constraints and
>> > exclusions, and in fact avoiding any such constraints and exclusions
>> > was a design goal of the entire project. This is because one expects
>> > that an RDF(S) reasoner will be using content expressed in RDF(S)
>> > which comes from a variety of sources, written by different authors
>> > at different times and possibly from completely different parts of
>> > the planet. To impose any kind of global constraint or exclusions
>> > would therefore be likely to prevent useful inferences being made,
>> > since it is impossible to guarantee that the merged pieces of RDF
>> > will satisfy any such constraint; and in any case there would be
>> > little point in doing so since there would be no central authority to
>> > report any such 'error' to.
>>
>>I tried to use these constraints to characterize the RDFS as an ontology 
>>language. It's also possible (maybe necessary) to give a subset of RDFS 
>>as an ontolofy language by constrainting the syntax. Suppose it's named 
>>as RDFS-ONT.
>>
>>In most use cases, people and machines just use RDFS-ONT and other 
>>ontology languages such as OWL Lite and OWL DL. They needn't to specify a 
>>new class of classes, a new class of properties, a new property about 
>>classes or properties. If they really need these constructs, they should 
>>define these constructs in a new ontology language and publish it.
>>
>>Suppose an RDF engine gets an RDF document containing a specification for 
>>a new class of classes, or a new class of properties. The RDF engine can 
>>do some really useful inferences?  For example, the RDF engine is not 
>>OWL-aware, and the class is OWL:TransitiveProperty (a class of 
>>properties). The useful inferences depend on the ontology language being 
>>used and the awareness of the engine.
>>
>>To impose some suitable constraints (as in RDFS-ONT) is to get more, 
>>including clearness, simpleness and performance. Yes, it doesn't prevent 
>>people from using the RDFS to specify a new class of classes, or a new 
>>class of properties. But, it's not really useful to do so.
>>
>>In sum, the another role of the RDFS is like RDFS-ONT, a constrained 
>>version of the RDFS. In addition, the (or similar) constraints should be 
>>satisfied by the other ontology languages.
>>
>> > >*Not allowed to define a new "meta-class" (class of classes).
>> >
>> > So any class of classes is a meta-class? That seems to me to be a
>> > very odd notion, particularly when there may be no way in general to
>> > tell if a class has other classes as its members.
>>
>>In this context,a "meta-class" means a class of classes. A class is a 
>>class of classes iff it's an rdfs:subClassOf rdfs:Class. The rdf:type 
>>relationship can be used to tell if a class is a member of a "meta-class" .
>>
>> > >  Examples of meta-classes:  rdfs:Class, rdfs:Datatype, OWL:Class
>> > >(This also implies that one couldn't use the rdfs:subClassOf
>> > >construct to specify a class from a predefined "meta-class")
>> > >
>> > >*Not allowed to define a new "property-class" (class of properties).
>> > >  Examples of property-classes:  rdf:Property,
>> > >rdfs:ContainerMembershipProperty, OWL:TransitiveProperty.
>> > >(This also implies that one couldn't use the rdfs:subClassOf
>> > >construct to specify a class from a predefined "property-class")
>> > >
>> > >*Not allowed to define a new property about classes or properties
>> > >(Typically, the rdfs:range and/or rdfs:domain of the property is
>> > >specified to be a "meta-class" or "property-class").
>> > >Examples:  rdfs:subClassOf, rdfs:subPropertyOf, rdfs:range, rdfs:domain,
>> > >OWL:equivalentClass
>> > >
>> > >*Not allowed to define a new subproperty of an RDFS Kernel
>> > >property(Or broaden to the predefined RDFS properties).
>> > >  The RDFS Kernel properties are as follows:
>> > >  rdfs:range, rdfs:domain, rdf:type, rdfs:subClassOf, rdfs:subPropertyOf.
>> > >  Other predefined RDFS properties includes rdfs:label, rdfs:comment, 
>> ....
>> > >
>> > >
>> > >Typically, An ontology language is used as follows:
>> >
>> > I am not sure what you mean by 'typically', but this does not
>> > correspond to the kind of ontology construction with which I am
>> > familiar. I do not think it would be appropriate for RDF to enforce
>> > this technique on all users.  However, I note that all the following
>> > can be done within the RDF(S) framework; that is, there is nothing in
>> > RDFS which *prevents* one from adopting this methodology. Do you
>> > agree?
>>
>>I agree.
>>
>>The previous constraints are using an exclusion approach. Here I use an 
>>inclusion approach.
>>I tried to figure out what constructs an ontology language (such as 
>>RDFS-ONTO) should provide:
>>
>>*meta-classes
>>*property-classes
>>*properties about classes or properties
>>
>>and how these constructs are used.
>>
>>By the previous constraints, I tried to figure out what mechanism an 
>>ontology language (such as RDFS-ONTO) should not provide: the mechanism 
>>to define a new class of classes, a new class of properties, a new 
>>property about classes or properties.
>>
>> > >*use the predefined "meta-class" to instantiate or specify a new class.
>> > >*use the rdfs:subClassOf construct to specify a class from a
>> > >user-defined class, or a predefined class other than a "meta-class"
>> > >or "property-class".
>> > >*use the predefined "property-class" to instantiate or specify a new 
>> property.
>> > >*use the rdfs:subPropertyOf construct to define a new property from
>> > >a user-defined property.
>> > >*use rdfs:range and/or rdfs:domain to constraint a user-defined
>> > >property,and the rdfs:range and/or rdfs:domain of the property must
>> > >not be a "meta-class" or "property-class".
>> > >
>> > >*specify individual resources.
>> > >*make assertions on individual resources
>> > >
>> > >*Other approach based on the specific constructs in the given
>> > >ontology language.
>> > >
>> > >
>> > >2. RDFS as a language for defining other (RDFS-based) ontology languages.
>> > >As a meta-language, RDFS is typically used to define the following
>> > >constructs of the target ontology language:
>> > >*"meta-classes" other than rdfs:Class. Example: OWL:Class
>> > >
>> > >*"property-classes" other than rdf:Property. Example: 
>> OWL:TransitiveProperty.
>> > >
>> > >*built-in core properties other than RDFS Kernel property
>> > >(rdfs:range, rdfs:domain, rdf:type, rdfs:subClassOf,
>> > >rdfs:subPropertyOf), they are properties about classes or
>> > >properties. Examples: OWL:equivalentClass, OWL:inverseOf.
>> > >
>> > >*Other constructs(classes, properties,individuals,...)
>> > >
>> > >
>> > >In sum, RDFS (as a meta-language) can be used to define other
>> > >ontology languages such as DAML and OWL, these languages are
>> > >extensions of RDFS (as an ontology language).
>> > >
>> > >The dual roles have different attitude to the following key constructs:
>> > >*meta-classes
>> > >*property-classes
>> > >*properties about classes or properties
>> > >
>> > >Roughly speaking, RDF Semantics (the spec) is ambitious in that it
>> > >tries to give the semantics of RDFS at both of ontology language
>> > >layer and meta-language layer with a single mechanism. It seems not
>> > >bad as the semantics of RDFS (as a meta-language), but not perfect
>> > >as the semantics of RDFS (as an ontology language) .
>> > >As an ontology language, RDFS should have a clear and fixed
>> > >semantics based on a subset of FOL (or other well known Logic such
>> > >as Order-Sorted Logic).
>> >
>> > I find it hard to reconcile this assertion (with which I fully agree,
>> > by the way) with your above description of the typical ontology
>> > language. FOL does not impose any of the constraints or restrictions
>> > which you describe above.
>> >
>> > >The semantics of OWL (as an extension of RDFS) can also be defined
>> > >by using the same approach.
>> > >
>> > >
>> > >Thanks for your concern, any comment is welcome!
>> >
>> > Thanks for your reply. I am not sure if we are converging on an
>> > agreement, but I certainly feel like I understand your ideas better.
>>
>>Thanks for your understanding, your response is very helpful!
>>
>>Yuzhong Qu
>>
>> > Pat Hayes
>> > --
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>> >
>> >
>
>
>
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>
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Received on Thursday, 20 February 2003 08:25:28 GMT

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