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Re: [SKOS] chatting on SKOS concepts and ontology classes (was Re: ISSUE-26: SimpleExtension proposal)

From: Daniel Rubin <rubin@med.stanford.edu>
Date: Thu, 31 May 2007 18:06:35 -0700
Message-Id: <6.2.5.6.2.20070531154103.02d76430@med.stanford.edu>
To: Antoine Isaac <aisaac@few.vu.nl>
Cc: SWD WG <public-swd-wg@w3.org>

At 10:31 AM 5/31/2007, Antoine Isaac wrote:
>Hi Daniel,
>>>
>>>>>>In the RadLex use case, for example, "terms" are entities in 
>>>>>>reality, such as blood vessels. These are linked together via 
>>>>>>relations such as "part-of" and "continuous-with".  It would be 
>>>>>>better for these things to be called "Entity" instead of "Term".
>>>>>>I think "Entity" would be consistent with terms and things in reality.
>>>>>I think your "entity" proposal, I fear this is too 
>>>>>ontology-oriented. SKOS exists to model knowledge organization 
>>>>>schemes, as very specific intellectual constructs made of 
>>>>>concepts (i.e. instances of skos:Concept), not entities in the 
>>>>>world themselves. For this real world things, ontologies should 
>>>>>be used, containing instances of owl:Class or rdfs:Class (notice 
>>>>>that OWL features an owl:Thing that pretty much corresponds to 
>>>>>the class of your "entities").
>>>>
>>>>I thought the goal of SKOS is to "provide a standard way to 
>>>>represent knowledge organization systems." Ontologies certainly 
>>>>fall under that umbrella. Some of the SKOS model is certainly 
>>>>very relevant to people building ontologies.
>>>Indeed. But assuming that a skos:Concept in a KOS actually denotes 
>>>entities in the world in a formal way - i.e. making it a class - 
>>>is an extra interpretation of your concepts.
>>>For a clearer account on this things just have a look at 
>>>http://www.w3.org/TR/swbp-skos-core-guide/#secmodellingrdf which 
>>>tries to explain this distinction.
>>
>>Perhaps it would be helpful if you provided the formal definition 
>>of "Concept." Is this the same thing as a "Term?" Clearly, there is 
>>a difference between actual things and the way we talk about things.
>OK, roughly copy-pasting from wikipedia. Not fully satisfied, but at 
>least we have a glimpse of the difference of level between the two notions:
>A concept is an abstract idea or a mental symbol, typically 
>associated with a corresponding representation in language or 
>symbology, that denotes all of the objects in a given category or 
>class of entities, interactions, phenomena, or relationships between them

So if I understand correctly, concepts are things in people's heads, 
NOT things that exist in reality.
Terms are labels for talking about either things in reality or concepts, yes?

>A "term" is a word, word pair, or word group, that is used in 
>specific contexts for a specific meaning.
>>
>>
>>>Notice however that if there is a modeling distinction, there is 
>>>no exclusion. You can have skos:Concepts (my:Car rdf:type 
>>>skos:Concept) that are also RDFS/OWL classes (ex:Car rdf:type 
>>>rdfs:Class) so that you can create 'objects' which are classified 
>>>under it (ex:danielsCar rdf:type ex:car). This would be needed by 
>>>a range of applications (including RadLex) that require using SKOS 
>>>features to define conceptual entities that are actually classes in ontologies.
>>
>>I'm confused by this statement. You say you can have skos:Concepts 
>>(my:Car rdf:type skos:Concept) that are also RDFS/OWL classes 
>>(ex:Car rdf:type rdfs:Class)--how can it be that a concept is also 
>>something tangible such as a car?
>I'm not saying that a concept can be tangible as a car, I'm saying 
>that a concept can be associated to a set of things (its extension), 
>that is, interpreted as a class (just as introduced in [1]).
>
>[1] http://www.w3.org/TR/rdf-schema/#ch_classes

Sorry, but I do not understand how a concept can be interpreted as a 
class. I can understand that skos:Concept can have relations to other 
things, such as rdfs:Class, but that is not the same thing as saying 
that the concept is interpreted as a class. Or perhaps I don't 
understand exactly what you are saying above.



>>>Notice that such a trick is needed because SKOS does not make any 
>>>assumption on the interpretations of instances of skos:Concept. 
>>>Before you say it is an RDFS/OWL class (eventually just by 
>>>creating an instance of it using rdf:type), your ex:Car is not 
>>>expected to denote a set of "entities" in a "real" world (i.e. a 
>>>concrete one, made of all kind of things like blood vessels and cars).
>>
>>I'm trying to understand this in terms of the relationship between 
>>skos:Concept and RDFS/OWL class. Are you saying that it is possible 
>>to say that owl:Class is-a skos:Concept?
>No. Here I want to say *an* owl:Class can be *a* skos:Concept 
>(ex:Car rdf:type owl:Class, ex:Car rdf:type skos:Concept)

Saying that *an* owl:Class can be *a* skos:Concept doesn't sound 
correct semantically to me. Can you give me an example of this?




>>>So to sum up my understanding, SKOS is meant to describe concepts, 
>>>as "something in someone's head".
>>>The point is that a concept can just stay in someone's head, or at
>>>the level of a (set of) manifestation in one language. This is the 
>>>level at which a skos:Concept would appear in a library, let's 
>>>say. They know what a car is (skos:definition "a car is something 
>>>that has four wheels, etc."), what it is semantically related to 
>>>(skos:broader ex:TransportMeans) and what labels are used to name 
>>>it in language (skos:prefLabel "car"@en). But their "world" do not 
>>>contain any concrete car.
>>
>>It's fine if skos wants to stay restricted to how people "talk 
>>about things", but there needs to be a formal way of relating that 
>>to ontologies that contain classes representing the things 
>>themselves and that also want to talk about how they are named. 
>>That's what's going on in RadLex.
>Ok, so I suppose that you have radlex:BloodVessel, which is an 
>instance of skos:Concept. And you want to say that you will have 
>ex:aorta as an instance of blood vessel, i.e. ex:aorta rdf:type 
>radlex:BloodVessel, don't you?

BloodVessel would be an instance of owl:Class. I don't know how you 
would relate this to skos:Concept.
Yes, you would say aorta is-a BloodVessel


>>>Then, if you have another world interested in car "entities" you 
>>>would have to make your car an RDFS/OWL class, that denote set of 
>>>concrete car entities (ex:danielsCar rdf:type ex:Car). But here we 
>>>enter the field of ontology engineering, and we don't want SKOS to 
>>>be redundant with RDFS/OWL
>>
>>But we need to be able to give ontology people a way to say in 
>>their ontology that their entities are named things, which is where 
>>skos comes in. It would not be semantically correct to do this by 
>>making owl Classes subclasses of skos:Concept.
>*instances of* skos:Concept (rdf:type skos:Concept) was what I 
>propose. *Not rdfs:subclassOf* skos:Concept, which is a different 
>thing, and surely wrong from a modeling point of view, I agree.

Then the big thing to clarify is how to handle OWL ontologies where 
you have instances of owl:Class--how to relate this to skos:Concept?

>Cheers,
>
>Antoine
Received on Friday, 1 June 2007 01:06:40 UTC

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