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RE: rdf:about and owl:sameIndividualAs oddities

From: Jimmy Cerra <jimbobbs@hotmail.com>
Date: Fri, 6 Jun 2003 01:46:49 -0400
To: "'pat hayes'" <phayes@ai.uwf.edu>
Cc: <public-webont-comments@w3.org>
Message-ID: <000501c32bef$05ce4c50$0100a8c0@picard>

Dear Dr. Hayes,

Your last comment, first:

> In view of the above, are you still interested in pursuing your
> suggestion?

No, your argument convinces me.  Thanks for reading my suggestion,
though.

--
Jimmy Cerra

] "I have learned these days, never to limit
]  anyone else due to my own limited
]  imagination." - Dr. Mae C. Jemison

> -----Original Message-----
> From: pat hayes [mailto:phayes@ai.uwf.edu]
> Sent: Thursday, June 05, 2003 12:45 PM
> To: Jimmy Cerra
> Cc: public-webont-comments@w3.org
> Subject: Re: rdf:about and owl:sameIndividualAs oddities
> 
> Dear Jimmy
> 
> Re. your comment
> http://lists.w3.org/Archives/Public/public-webont-
> comments/2003May/0003.html
> 
> I'd like to try to clarify some aspects of the OWL and RDF design, by
> responding to some of the points in your message:
> 
> >>I've been reviewing OWL, and a strange idea occurred to me.  In rdf,
> >>"blank nodes" are distinct resources.  However, the "thing" that
they
> >>are describing is undefined [1].
> 
> Er..that is rather a nonstandard way of putting it and may be
> misleading. They do not identify any particular thing, true; but its
> not that they are describing a particular undefined thing, more that
> they are noncommittal about what the thing is. Think of a blank node
> as being like the words 'something' or 'someone' in English. If you
> say, "Joe wrote #foobar"  then you aren't exactly contradicting or
> denying the claim "Joe wrote something", you are just saying more
> about it. Notice that it would be a logical mistake to conclude from
> "Joe wrote something" that there was a special 'blank' thing called
> "something" that Joe had written.
> 
> >>The following is an example:
> >>
> >><rdf:Description>
> >>	<dc:creator>Roy G. Biv</dc:creator>
> >></rdf:Description>
> 
> Something has Roy as its dc:creator, ie Roy wrote something.  OK.
> 
> >>However, using OWL, I could identify the resource by identifying a
> >>resource which is the same as the blank one:
> 
> Well, the locution "the blank one" isnt strictly correct, as there
> isnt a single 'blank one'. Better to say, you could find out more
> about the resource that you were previously unable to identify with
> precision.  For example, you might discover later what it was that
> Joe wrote, and add that information to your OWL knowledge, and then
> be able to draw more useful conclusions (about Joe, for example). In
> general, you might find out anything: for example, you might go
> through finding out that it was a book he wrote (but you don't know
> which book) , that the book that he wrote had a co-author called Bill
> J. Hickory (you still don't know which book) and later that Bill J.
> Hickory is the author of only one book, called "The perils of Ruth" -
> now you know what book Joe wrote. Only the last stage would enable
> you to replace a blank node with an actual name.
> 
> >><rdf:Description>
> >>	<owl:sameIndividualAs rdf:resource="#foobar" />
> >>	<dc:creator>Roy G. Biv</dc:creator>
> >></rdf:Description>
> >>
> >>However, the meaning of the above statements is equivalent to a
> >>"non-blank" node describing "#foobar", as in:
> >>
> >><rdf:Description rdf:about="#foobar">
> >>	<dc:creator>Roy G. Biv</dc:creator>
> >></rdf:Description>
> 
> Right. In other words, if you already know what it was that Joe
> wrote, it is kind of redundant to also say that he wrote something.
> The two forms above are roughly like saying in English
> 
> Joe wrote something which was #foobar
> 
> and
> 
> Joe wrote #foobar
> 
> which amount to saying the same thing, in effect, since you can infer
> either of them from the other.
> 
> >>One could theoretically define the identity of all objects in that
> >>manner.
> 
> Not sure what you mean. Certainly, it would be possible to take any
> OWL triple (Using Ntriples notation):
> 
> ex:foo ex:Property ex:baz
> 
> and replace it with
> 
> _:x ex:Property ex:baz
> _:x owl:sameIndividualAs ex:foo
> 
> and the two are equivalent. And since they are, the second form is
> kind of confusing (not wrong, but confusing) since one might easily
> think, when reading it, why didn't they just write the first one?
> 
> The chief utility of owl:sameIndividualAs is not for writing out
> facts like the above in a more long-winded form, but for being able
> to *conclude* - infer -  that two names are names of the same
> individual (eg because they are both values of a functionalProperty
> applied to the same thing, or both subjects of an
> InverseFunctionalProperty with the same value). And since this kind
> of inferring is so useful, it would be a mistake to deprecate it.
> 
> >>However, that would be confusing.  Should identifying blank
> >>resources with OWL, instead of RDF, be depreciated?
> >>
> >>I think that having two mechanisms for identifying a resource is
"messy"
> 
> It might be messy, but such mess is useful. The central idea of OWL
> on the SW is to support useful inferences, and inferring new facts is
> inherently messy in this sense because you can't always pre-guess
> *how* exactly the facts are going to be generated. Often it is best
> to allow for alternative ways to the same conclusion because one of
> them might be easier to find than another, or some of them might be
> impossible to use in some circumstances.
> 
> >>and makes the grammar combination of RDF+OWL inconsistent.
> 
> It doesn't make the grammar inconsistent. There is some redundancy,
> but as I explain above, this kind of messiness is actually useful,
> even essential, in an inferential language. OWL is highly redundant
> in this sense: often the 'same' thing can be said in  many different
> ways.
> 
> >>Except for
> >>different ranges, owl:sameIndividualAs (range=resource) and
rdf:about
> >>(range=string) mean the same thing.  The extra syntax seems
redundant.
> 
> I think there is a slight category mistake here. rdf:about is part of
> the RDF/XML syntax; it isn't a property in itself: rather, it is what
> attaches a property to the subject of a triple (or triples). So it
> doesn't really have a range in the RDFS sense. sameIndividualAs, on
> the other hand, is a genuine property, which can be used not only to
> attach a name to a blank node but also has many other uses, since it
> can be used to express identity between URIrefs.
> 
> >>If rdf:about was an actual property, then the two concepts could be
> >>combined.
> 
> I see what you mean, but this would allow the 'combining' to be done
> only in this particular use case, which is not the primary intended
> use for owl:sameIndividualAs (and not really a very interesting use
> case, in fact); and only in the XML syntax, which is only one of the
> possible concrete syntaxes for RDF. And moreover, this *would* make
> the RDF grammar inconsistent, since rdf:about would then have two
> incompatible uses (as your example below illustrates, in fact.)
> 
> >>Furthermore, if the property-as-attribute abbreviation for
> >>rdf:type was applied to rdf:about as well, then the abbreviated
syntax
> >>would be identical to the 1999 xml-serialization syntax [2].
However,
> >>this would still allow the equivalent owl:sameIndividualAs
> >>representations, such as the below example:
> >>
> >><rdf:Description rdf:about="http://www.w3.org/TR/html4/strict.dtd">
> >><rdf:about rdf:resource="urn:publicid:-:W3C:DTD+HTML+4.01:EN" />
> >></rdf:Description>
> >>
> >>The above seems much more intuitive than the OWL version:
> >>
> >><rdf:Description rdf:about="http://www.w3.org/TR/html4/strict.dtd">
> >><owl:sameIndividualAs
rdf:resource="urn:publicid:-:W3C:DTD+HTML+4.01:EN"
> >>/>
> >></rdf:Description>
> >>
> >>Or even:
> >>
> >><rdf:Description>
> >><owl:sameIndividualAs
> >>rdf:resource="http://www.w3.org/TR/html4/strict.dtd"/>
> >><owl:sameIndividualAs
> >>rdf:resource="urn:publicid:-:W3C:DTD+HTML+4.01:EN"/>
> >></rdf:Description>
> >>
> >>Just an idea [3].
> 
> In sum: owl:sameIndividualAs is basically the OWL version of
> equality. You point out (correctly) that there is little point in
> writing things like 'x property baz and x = foo' when you could just
> as well write 'foo property baz'; but the moral isn't to forbid or
> deprecate the use of equality, which has many other uses as well as
> this rather redundant way of expressing a simple fact.
> ----------
> 
> In view of the above, are you still interested in pursuing your
> suggestion?
> 
> Thanks.
> 
> Pat Hayes
> 
> 
> --
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Received on Friday, 6 June 2003 01:46:59 GMT

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