- 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>
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 > > > -- > --------------------------------------------------------------------- > IHMC (850)434 8903 or (650)494 3973 home > 40 South Alcaniz St. (850)202 4416 office > Pensacola (850)202 4440 fax > FL 32501 (850)291 0667 cell > phayes@ai.uwf.edu http://www.coginst.uwf.edu/~phayes > s.pam@ai.uwf.edu for spam
Received on Friday, 6 June 2003 01:46:59 UTC