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Re: review of LCC documents as of 26 December 2002

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 02 Jan 2003 10:03:52 -0500 (EST)
Message-Id: <20030102.100352.93843460.pfps@research.bell-labs.com>
To: bwm@hplb.hpl.hp.com
Cc: www-rdf-comments@w3.org

From: Brian McBride <bwm@hplb.hpl.hp.com>
Subject: Re: review of LCC documents as of 26 December 2002
Date: Thu, 02 Jan 2003 13:53:34 +0000

> > > >RDF Schema is not described in terms of RDF.  This error, which has been
> > > >made in several other places, must be ruthlessly eradicated.  Instead, RDF
> > > >Schema is a semantic extension to RDF.
> > >
> > > Hmm, I'm not sure that the intended audience for this document will
> > > appreciate the difference.  Whilst I don't want to say anything that is
> > > technically incorrect,  this document also needs to avoid over reliance on
> > > understanding model theory.  I've modified the passage in question to:
> > >

> > > [[
> > > RDF's vocabulary description language, RDF Schema, is an extension of
> > > RDF.  

This statment is true, but extremely misleading, particularly given past
documents that have failed to make a distinction beDublin Coretween things like RDF
Schema and things like the representation of Dublin Core in RDF(S).

> > > All RDF vocabularies share some basic common structure: they describe
> > > classes of resource and types of relationships between resources. 

There are several problems with this statement.  The only definition of a
vocabulary is in Semantics, where a vocabulary is defined as a set of URI
references.   A set of URI references, by itself, does not describe

Even if the term ``RDF vocabulary'' meant either a semantic extension to
RDF (like RDFS) or an RDF(S) document (like the RDF(S) representation of
the Dublin Core) the statement would not be true.  Such extensions or
representations need not be about classes or types of relationships.  For
example, it would be useful to create a semantic extension of RDF that
defined special semantic conditions for a particular resource or property;
it would also be useful to create an RDF document that served simply to
provide a common set of URI references.

> > > RDF
> > > Schema uses RDF graphs to describe these classes of resources and
> > > relationships.

This statement is incorrect.  RDF Schema does not use RDF graphs to
describe.  RDF Schema uses a semantic extension of RDF to provide an
ability to describe classes or resources and relationships.

> > > ]]

> >This is still unacceptable to me.  The RDF documents need some way of
> >distinguishing semantic extensions (like RDFS) from things that fit within
> >RDF or RDFS (like the formal part of the Dublin Core).  RDF Core needs to
> >make this distinction perfectly clear to readers.
> Perfection is something we strive for, yet rarely achieve.

Well, perhaps, the RDF Core documents do not have to be perfect.

However, they should at least help comprehension of RDF and RDFS, or,
maybe, not harm such comprehension.  I believe that the above paragraph
firmly sits in the category of paragraphs that will end up harming the
comprehension of RDF and RDFS.

> My current understanding of your objection is:
>   o that the new text does not say anything that is false

Part of my objection is that the new text does say false things.

>   o the new text does not state something you consider it important to make 
> clear

Part of my objection is that this paragraph, above all others in all the
RDF Core WG documents, needs to make a distinction between RDF semantic
extensions and RDF graphs.

>   o that which you desire be made clear is a technical issue for semanticians

Part of objection is that this distinction is vital for the effective use
of RDF.

>   o that which you desire made clear is made clear in the semantics doc

This distinction is, correctly, not even addressed in the semantics

> If that understanding is correct, I see no problem, so I presume my 
> understanding is incorrect.  Please help me understand.

See above.

> > > >This document again makes the claim that RDF ``allows anyone to extend the
> > > >description of existing resources, one of the architectural principles of
> > > >the Web''.  Again, would that this were so.  There are many statements in
> > > >the RDF documents and being currently made in the RDF Core WG that
> > > >contradict this claim.
> > >
> > > Just as it takes two to tango, it takes at least two to contradict.  Can
> > > you help me out here with a more specific objection.
> >
> >The discussion concerning allowing ``owners'' of resources to control their
> >use runs counter to this claim.
> What discussion?  This is what URL's are for.

Lots of recent discussion in the RDF Core WG mailing list has concerned how
to let owners of namespaces control how they are used.

> >   Also the Concept document says that
> >``[c]ertain URIs are reserved for use by RDF, and may not be used for any
> >purpose not sanctioned by the RDF specifications''.
> I don't see a contradiction there.

How can this not be a contradiction with the desire to allow anyone to
arbitrarily expand upon the meaning of existing resources?

> >[..]
> >
> > > >Section 2:
> > > >
> > > >Property is a technical term in RDF and should not be used for other
> > > >purposes.
> > >
> > > I'm not sure what text you are objecting to here.
> >
> >``Only one of these classes has the property that it was defined by the tax
> >office, ....''
> Right,  and the term 'property' is correctly used in the technical sense to 
> which you refer.

Well, I sure read it that the ``defined by the tax office'' meant ``defined
by the tax office', not ``having a "defined" property whose value is "tax
office"''.  I consider these two senses of property to be very different.


> > > >Section 3:
> > > >
> > > >The RDF semantic does not support the inference that properties with no
> > > >rdfs:range or no rdfs:domain have a range or domain of rdfs:Resource.  In
> > > >any case, this would be a non-monotonic inference as stated.
> > >
> > > Are you referring to, in 3.1
> > >
> > > [[In this specification, where no rdfs:range is specified for a property,
> > > the rdfs:range of that property is rdfs:Resource.]]
> > >
> > > and in 3.2
> > >
> > > [[In this specification, where no rdfs:domain is specified for a property,
> > > the rdfs:domain of that property is rdfs:Resource.]]
> > >
> > > This was an editorial judgement.  The intent here is that, in the 
> > interests
> > > of brevity, the text omitted statements that the domain and range of
> > > properties were rdf:Resource.  The summary table however, and the rdf/xml
> > > representation of the schema has these in full.
> > >
> > > No inference is stated or implied.
> >
> >How can you say this?  If no rdfs:domain is specified for a property, then
> >there is *no* rdfs:domain for that property.  To say otherwise is just
> >incorrect!
> Its all to do with layering.

It has nothing to do with layering.  It has, instead, to do with a
particular point of the RDF model theory, which has been the subject of
considerable debate.  I happen to not like the stance on this issue
utilized in RDF, but the RDF Core WG documents should at least abide by
this stance.

> > >
> > > However, if this is not clear, it would be possible, though tedious, to go
> > > back and add in the text.
> > > Are you sure this is necessary?
> >
> >Yes, YES ***YES***!
> OK.
> [...]
> > > >It is more than misleading to say that RDF Schema does not specify how
> > > >domains and ranges are to be used.  The RDF semantic states precisely how
> > > >domains and ranges are to be used.
> > >
> > > Oh?  Where exactly?  We have tried to be careful to stay away from any 
> > sort
> > > of processing model.  I grant you that RDF Semantics specifies what
> > > entailments are justified, but that is not the same thing as how they 
> > might
> > > be used.
> >
> >The RDF semantics states that domains and ranges are used to eliminate
> >certain interpretations what might otherwise be valid.  (The use of a
> >notion is not tied to a processing model.)
> The current text says:
>    [[but does not say whether or how an application should use it.]]
> The applicable concept here is not "use", but "application use".  Whilst to 
> you, an applications using a vocabulary for data validation, for inference 
> and for constructing a useful user interface might all be doing the same 
> thing, i.e. "restricting the set of interpretations", I don't agree that 
> language is helpful in making the point we are trying to make.
> I believe the meaning of the text will be clear to its intended audience, 
> an opinion I am willing to revise in the light of other feedback to the 
> contrary.
> >[...]
> >
> > > >  and these other things may
> > > >even be reasonable extensions of RDF, but any allusion to such activities
> > > >needs to have a very strong disclaimer attached to it.
> > >
> > > This section is present to deal with a common issue raised about RDF.  The
> > > term 'schema' tends to be associated with validation and this has led to
> > > much confusion. All this section is saying that domain and range may be
> > > used in different ways by different applications.  It is important to 
> > say that.
> >
> >It is important *not* to say that in a place or manner that might cause it
> >to be construed as part of the definition of RDFS.  Because this document
> >has some sort of normative status, great care should be taken not to put
> >more into RDFS than is actually there.
> Would marking the section as informative rather than normative help?

Perhaps.  I would have to revisit this issue in the context of the entire

> [...]
> Brian

Received on Thursday, 2 January 2003 10:05:53 UTC

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