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Re: [ontolog-forum] RDF & RDFS (was... Is there something I missed?)

To: edbark@xxxxxxxx, "[ontolog-forum] " <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Pat Hayes <phayes@xxxxxxx>
Date: Tue, 3 Feb 2009 13:01:42 -0600
Message-id: <8FA5F746-B282-4518-AE91-B90421F3FE8E@xxxxxxx>

On Feb 2, 2009, at 4:30 PM, Ed Barkmeyer wrote:    (01)

> Ian Bailey wrote:
>> The SWeb, as far as
>> I can deduce, is doing for data what HTML did for hypertext...i.e.  
>> allowing
>> relationships to go outside the confines of the computer the data  
>> sits on.
> Now there's a definition. :-o
>> If you treat RDF and RDFS as simple modelling syntaxes (that's all  
>> they
>> are), you can use them quite nicely to represent your ontology -  
>> even an
>> extensional, 4D, higher-order ontology. It's less easy to do that  
>> with OWL,
>> as that language tries to be a bit more clever (and as always with  
>> things
>> that try to be too clever, shoots itself in the foot). It's just a  
>> syntax -
>> the semantics should be in your ontology, not in the language you  
>> choose to
>> publish it in.
> This is false, and it misses an important point.
> RDF introduces a minimal modeling vocabulary, which allows the
> expression of essentially arbitrary relations and logical statements.
> OWL/DL introduces a much richer vocabulary, but removes certain RDF
> vocabulary elements that allow the expression of arbitrary logical
> statements (e.g., "implies").  What that does is to limit the kinds of
> statements you can make in OWL.  (And those limitations guarantee that
> certain reasoning algorithms will terminate in boundable time, which  
> is
> not true of RDF.)    (02)

For the record, this isn't exactly right, though its right in spirit.  
RDF is actually much less expressive than any version of OWL, and has  
extremely fast trivial reasoning algorithms (basically, just compute  
the logical closure and then look up the answer by triple matching.)  
But RDF allows for 'semantic extensions' which are RDF syntax with  
extra semantic conditions added. RDFS is one, and it is still pretty  
limited in expressivity (neither RDF nor RDFS can express logical  
"implies" or the universal quantifier, for example) and hence still  
tractable: Cicso implemented an RDFS reasoning engine using the  
inference rules published in the RDF semantics document, for example,  
which runs at industrial speeds. But the next RDF semantic extension  
is OWL-Full, and now this really is rather expressive and not  
decideable any more (though its still not full FOL.) And OWL-DL bears  
the relationship that Ed describes to OWL-Full: its a syntactic  
limitation or restriction of it, in which many constructions are  
declared illegal, and this is done in order to make the inference  
problem tractable, to bring it into the description-logic umbrella  
category.    (03)

> In each case, the base vocabulary has a (strongly)
> specified semantics and that semantics enables the interpretation of
> sentences in the language.  It is not "just a syntax".    (04)

Agreed. However, the 'pure' RDF formal semantics is so minimal that  
anyone might be forgiven for treating it as nonexistent. And certainly  
Ian is free to impose extra semantics onto any RDF vocabulary he  
wishes to use, provided of course that it does not clash with the RDF  
meaning it already has[1] and the RDF spec sanctions this. Of course,  
he ought to explain it (his extra semantics) somewhere, but that is  
just being a responsible citizen.    (05)

Pat    (06)

[1] Example of such a clash might be saying that blank nodes are  
universally quantified, or saying that some URI means logical  
disjunction. Which is why getting rules into RDF syntax isn't easy or  
trivial.    (07)

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