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

From: Antoine Isaac <aisaac@few.vu.nl>
Date: Fri, 08 Jun 2007 10:36:23 +0200
Message-ID: <46691507.8020504@few.vu.nl>
To: Daniel Rubin <rubin@med.stanford.edu>
CC: SWD WG <public-swd-wg@w3.org>

Daniel Rubin a écrit :
> At 08:42 AM 6/6/2007, Antoine Isaac wrote:
>> Hi Daniel,
>>>>> 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.
>> Let's say yes (because of course I consider that everything which 
>> happens in my head and that I communicate to others has some form of 
>> reality ;-)
>>> Terms are labels for talking about either things in reality or 
>>> concepts, yes?
>> Mmmh, yes and no. If you look closely at the quotation I made:
>>>> A "term" is a word, word pair, or word group, that is used in 
>>>> specific contexts for a specific meaning.
> Yes, but that "meaning" refers to what--a concept, or anything that 
> word(s) represent? ie, this definition seems to be describing words in 
> a dictionary. For example, "at the" would qualify as a "term" by your 
> definition.

You're right. The definition I quoted is imprecise (but how to explain 
term in one sentence? I clearly don't have time to review all textbooks 
for that now). So let's consider, taking your words, that "meaning" 
refer to some concept. And that this is quite closely related to what 
happens in a dictionary, indeed.

> I had always thought that thesauri comprised terms that referred to 
> "things" that people describe in a domain of discourse, such as 
> "bookstore," etc.

Yes. But be careful: thesauri do dot mandatorily comprise concepts that 
refer to things people describe: in a library (and many other places 
where one uses a thesaurus), a thesaurus will include a "car" subject, 
but nobody will want to describe cars there. Notice that this is an 
important difference with ontologies: if you have a "car" class in an 
ontology, it is indeed expected to be used in description of cars.

>> you can see that there is slightly more to a term than just a label. 
>> This "meaning load" is actually what allowed people from thesaurus 
>> community to speak about "broader term" and "related term" for links 
>> that in SKOS we say to exist between concepts (skos:broader and 
>> skos:related)
>> This is the reason why I want to avoid the use of "term" in Guus' 
>> proposal about relationship between label. I prefer the more neutral 
>> "lexicalization" (lexicalization does only say that the word is 
>> related to some concept, not that it carries a part of meaning with it)
> This sounds to me like you are describing *language* and not *things* 
> that people want to talk about in a domain of discourse.

Cf my remark above. Yes, SKOS is mainly focused on what you call the 
language level, because it does not assume the existence of instances in 
real world for the concepts defined with it. There is not even a 
beginning of a problem when introdcuing "unicorn" as a SKOS concept, for 

> To me, lexicalization is about the way tokens can appear, but what 
> they *represent* is what belongs in the thesauri, represented as 
> "terms" (how they can appear in languages could be a property of the 
> term).

Yes, this analysis of lexicalization as tokens (I call them labels) is 
pretty fitting what I wanting to use, even clarifying it ;-)

Bout don't forget that what thesauri traditionally call "terms" are in 
SKOS approach to be represented as instances of skos:Concept! In Current 
SKOS there is nothing between the concept and its appearance in language 
(label). And what traditional thesauri denote as term relationship (that 
are indeed conceptual relationship, as "broader term") is modelled in 
SKOS using concept links (skos:broader)

To add to this mutual clarification process: as said, nothing like a 
"term" appears now in current SKOS, it only popped up in the discussion 
because of loosely wording used in the proposals for solving the 
RelationshipBetweenLabels issue. [and of course I share a great deal of 
responsability for that :-(] Hence my will to replace"term"by 
"Lexicalization" (or by anything more neutral than "term") in Guus' 

>>>>>> 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.
>> Well, a concept (understood here as the general 'concept' notion, not 
>> only SKOS one) can have an intensional interpretation (its 'meaning', 
>> definition: "has 4 wheels etc.") and an extensional interpretation 
>> (the objects it categorizes: my car, your car, the neighbour's car).
>> It is true that SKOS deals only with the first aspect: it does not 
>> anticipate what could be the instances of a skos:Concept, and its 
>> definition belongs primarily to the informal realm (text definitions, 
>> labels, semantic links which interpretation can be rather fuzzy like 
>> 'broader')
>> On the other hand, OWL deals primarily with the extensional aspects. 
>> Semantics of class are indeed based on the possible instances of this 
>> class. For example, if you say that the class Car is a subClassOf 
>> class TransportationMeans, you say that all instances of the first 
>> (concrete cars) are instances of second.
> And what I am saying is that there is a community of people creating 
> OWL ontologies who want to use skos for interoperability with 
> terminologies.

I think that even if this is the direction opposite to the one I 
demonstrated, this is still quite the same concern. If you have
my:aorta rdf:type owl:Class
you can just assert
my:aorta skos:prefLabel "aorta"
And bang, it is now also an instance skos:Concept, compatible with other 
terminologies. You can say my:aorta skos:broader his:BloodyThingsInbody, 
assuming that this is a concept define in someone else's terminology.

>>>> 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?
>> I agree that it sounds weird, but nothing actually prevents it!
>> Look at the class:
>> my:Car [ a owl:Class; rdfs:subClassOf my:TransportationMeans]
>> You can actually add a SKOS triple like
>> my:Car skos:prefLabel "car"
>> And then, by virtue of RDFS semantics (and because the rdfs:domain of 
>> the skos:prefLabel property is skos:Concept) it also becomes a 
>> skos:Concept!
> Perhaps we should have skos:Entity then in skos to avoid such problems?

1. Personnally I don't see this as a problem. It is more a feature, 
illustrating how SKOS and OWL can live together when it is needed.

2. Trying to clarify your proposal: skos:Entity would refer to the 
concepts that denote "real things", in the sense that they have 
instances, and therefore can be considered as classes in ontologies?
And you would have something like skos:Entity rdfs:subClassOf 
skos:Concept? With some rule saying that all instances of skos:Entity 
are also instances of owl:Class?
I don't feel it's a necessary step (because there is compatibility 
between OWL and SKOS, as I've demonstrated) but if you feel that this 
could make things a lot clearer for users of SKOS interested in 
ontologies (or vice versa) then I *warmly welcome it* as a proposal and 
am ready to help you formalizing it.


>> This trick has the disavantage to make your knowledge base OWL full: 
>> you have an instance of an OWL class (my:Car, as an instance of 
>> skos:Concept) which is considered as a class (my:Car as an instance 
>> of owl:Class).
>>>>> 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.
>> [snip]
>>> 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?
>> I hope to have demonstrated at least that this was possible.
>> Be aware however that I played the devil's advocate here, and that I 
>> would not generally advocate this solution, unless in specific cases, 
>> those where SKOS concepts can be associated to these very precise 
>> class interpretation.
>> Especially I think it does not apply to all the cases where the SKOS 
>> concepts should only be used in statements involving 
>> skos:subject-like properties ("this book is about blood vessels") and 
>> not be used in rdf:type-like statements ("aorta is a blood vessel").
> Ideally, skos should permit both types of statements.
>> Cheers,
>> Antoine
Received on Friday, 8 June 2007 08:36:27 UTC

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