Hi Simon, yes, that is a neat summary of the matter.
In the background of this is that Dublin Core originated before RDF and was later retrofitted.
A very readable and interesting overview of this which I didn't know until I stumbled upon it this morning is [1].
A (normative) paper explaining in more detail how Dublin Core translates to RDF is [2], which ominously writes:
"The property dcterms:type has semantics very similar to rdf:type. At the time of writing, the precise relationship between those properties remains undecided.
It is recommended that RDF applications implementing this specification primarily use and understand rdf:type in place of dcterms:type when expressing Dublin Core metadata in RDF, as most RDF processors come with built-in knowledge of rdf:type."
Combining this insight with the original "abstract" notion of type, which is more like "genre", it seems reasonable to drop the range declaration... -j
[1] http://wiki.dublincore.org/index.php/Review_of_DCMI_Abstract_Model
[2] http://dublincore.org/documents/dc-rdf/
________________________________________
Van: DCMI Architecture Forum <[log in to unmask]> namens Simon J D Cox <[log in to unmask]>
Verzonden: donderdag 13 augustus 2015 06:41
Aan: [log in to unmask]
Onderwerp: Re: dcterms:type and SKOS
Hi Jan -
Ok - I think I understand a bit better. The issue is not so much the SKOS meta-modelling capability, but the formal definition of dcterms:type. As you point out, if
dcterms:type rdfs:range rdfs:Class .
then it is rather too similar to rdf:type!
In the RDF universe, rdf:type is a special predicate, being the only one that is routinely used to cross meta-levels, relating instances to types (classes). The formal semantics of dcterms:type appears to duplicate rdf:type, and precludes the use of a skos:Concept as its value because of the risk of breaking DL semantics.
Simon
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Date: Wed, 12 Aug 2015 06:05:21 +0000
From: Jan Voskuil <[log in to unmask]>
Subject: Re: DC-ARCHITECTURE Digest - 10 Aug 2015 to 11 Aug 2015 (#2015-26)
Hello Simon,
The problem you are referring to is related to but slightly different from the dcterms:type-issue.
I agree that a given skos:Concept often "refers to" or "is about" something that may be thought of as a class or set.
Note that "refers to" here is used not in the rdf-semantics sense of the word. The notion of aboutness causes instances of skos:Concept to function as a kind of metamodeling-vocabulary.
In my current project we use instances of skos:Concept to make "first class modeling concepts" in our vocabularies easily accessible and findable. To do this, we use foaf:focus to relate the concept in the thesaurus to the class, property or individual in our ontology. Example:
ex:OrganisationConcept a skos:Concept ;
foaf:focus rov:RegisteredOrganisation.
Interesting literature on this is [1], [2] and [3].
Note, however, that in the above example individuals and classes are strictly separated. The construction satisfies strict DL-constraints. One problem with dcterms:type is that it introduces punning and is (to some undefined degree) synonymous to rdf:type, independently of the metamodeling-features of SKOS. The other problem is that as a result of this, people are hesitant to use skos:Concept as the value of this property. -Jan
[1] http://xmlns.com/foaf/spec/#term_focus
[2] “Getty Vocabularies: Linked Open Data Semantic Representation”, chapter 3 “Concept vs Thing Duality”, http://vocab.getty.edu/doc/#Concept_vs_Thing_Duality.
[3] Pete Johnston, “Things & their conceptualisations: SKOS, foaf:focus & modelling choices”, http://efoundations.typepad.com/efoundations/2011/09/things-their-conceptualisations-skos-foaffocus-modelling-choices.html
________________________________________
Van: DCMI Architecture Forum <[log in to unmask]> namens Simon J D Cox <[log in to unmask]>
Verzonden: woensdag 12 augustus 2015 01:23
Aan: [log in to unmask]
Onderwerp: Re: DC-ARCHITECTURE Digest - 10 Aug 2015 to 11 Aug 2015 (#2015-26)
Dear Jan -
This is actually a well-known problem/feature (depending on your pov) in SKOS.
SKOS Concepts are often 'classes' in the generic sense, but the SKOS RDF vocabulary makes them instances.
SKOS is really a bridging vocabulary to help move traditional 'vocabularies' into RDF, but stops short of modelling them fully as classes.
Your reference to 'punning' is on point. Don't expect DL compliance.
Simon Cox
Simon J D Cox
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Date: Tue, 11 Aug 2015 16:55:54 +0100
From: Jan Voskuil <[log in to unmask]>
Subject: dcterms:type and SKOS (retried)
Dear all,
In recent work I have struck on a problematic interaction between SKOS and DC. These standards should strengthen each other. Using SKOS to publish value lists and then to use the skos:Concepts therein as the value of, for instance, dcterms:subject, offers significant benefits. However, there is a problem with dcterms:type, because it is declared with rdfs:Class as its range. This complicates the use of instances of skos:Concept as its value.
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