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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I appreciate the time you're taking to unpack this point. I agree that it's subtle and important. Speaking of prescription and description, I suspect that we're "talking past each other" along those lines. I understand you to be expositing the prescribed use of OWL, and I'm describing one of the uses to which I see it being put today. I also suspect that I've been making this more difficult by using language loosely. When I used the word "validation", I didn't mean simply the functionality of accepting or rejecting an instance based on a set of rules, especially not on syntactic grounds. I meant the discovery of inconsistencies. For example, you say: >> If you say that every resource MUST have a dct:title, then if you come across a resource without a dct:title that is NOT wrong. The reasoner would conclude that there is a dct:title somewhere because that's the rule. (This is where the Open World comes in) When data contradicts reasoners, they can't work correctly, but they act on existing data, they do not modify or correct data. and without disagreeing with you, I would point out that if you say that a given type of resource can have at most one dct:title (which is easy to declare using OWL), and then apply that ontology to an instance that features a resource of that type with two dct:titles, you're going to get back useful information from the operation of your reasoner. An inconsistency in your claims about the world will become apparent. I now realize I should have been using the word "consistency" and not "validity". I suppose what I really want to know, if you're willing to keep "playing reporter" on the workshop you attended, is whether there was an understanding present that people are using OWL in this way, and that it's useful in this way (far more useful than writing and maintaining lots and lots and lots of SPARQL) and that this is a use case for ontology languages. - --- A. Soroka The University of Virginia Library On Sep 17, 2013, at 11:00 PM, CODE4LIB automatic digest system wrote: > From: Karen Coyle <[log in to unmask]> > Date: September 17, 2013 12:54:33 PM EDT > Subject: Re: Expressing negatives and similar in RDF > > > Agreed that SPARQL is ugly, and there was discussion at the RDF validation workshop about the need for friendly interfaces that then create the appropriate SPARQL queries in the background. This shouldn't be surprising, since most business systems do not require users to write raw SQL or even anything resembling code - often users fill in a form with data that is turned into code. > > But it really is a mistake to see OWL as a constraint language in the sense of validation. An ontology cannot constrain; OWL is solely *descriptive* not *prescriptive.* [1] > > Inferencing is very different from validation, and this is an area where the initial RDF documentation was (IMO) quite unclear. The OWL 2 documents are better, but everyone admits that it's still an area of confusion. (In a major act of confession at the DC2013 meeting, Ivan Herman, head of the W3C semantic web work, said that this was a mistake that he himself made for many years. Fortunately, he now helps write the documentation, and it's good that he has that perspective.) In effect, inferencing is the *opposite* of constraining. Inferencing is: > > "All men are liars. Socrates is a man. Therefore Socrates is a liar." > "Every child has a parent. Johnny is a child. Therefore, Johnny has a parent." (whether you can find one or not is irrelevant) > "Every child has two parents. Johnny is a child. Therefore Johnny has two parents. Mary is Johnny's parent." (no contradiction here, we just don't know who the other parent is) > "Every child has two parents. Johnny is a child. Therefore Johnny has two parents. Mary is Johnny's parent. Jane is Johnny's parent. Fred is Johnny's parent." Here the reasoner detects a contradiction. > > The issue of dct:titles is an interesting example. dct:title takes a literal value. If you create a dct:title with: > > X dct:title http://example.com/junk > > with OWL rules that is NOT wrong. It simply provides the inference that "http://example.com/junk" is a string - but it can't prevent you from creating that triple, because it only operates on existing data. > > If you say that every resource MUST have a dct:title, then if you come across a resource without a dct:title that is NOT wrong. The reasoner would conclude that there is a dct:title somewhere because that's the rule. (This is where the Open World comes in) When data contradicts reasoners, they can't work correctly, but they act on existing data, they do not modify or correct data. > > I'm thinking that OWL and constraints would be an ideal training webinar, and I think I know who could do it! > > kc > > [1] http://www.w3.org/TR/2012/REC-owl2-primer-20121211/ > "OWL 2 is not a schema language for syntax conformance. Unlike XML, OWL 2 does not provide elaborate means to prescribe how a document should be structured syntactically. In particular, there is no way to enforce that a certain piece of information (like the social security number of a person) has to be syntactically present. This should be kept in mind as OWL has some features that a user might misinterpret this way. " -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.0.19 (Darwin) Comment: GPGTools - http://gpgtools.org iQEcBAEBAgAGBQJSOam0AAoJEATpPYSyaoIkGpoIAIsIMO+Ev2d/vdru8O9fQdz0 v770CxK1Dh/x3GHY9HO7mrbEBpF2IoEWfhuC5UfUunpaKUBybSCmngu9gBelRm59 AmPA6FAP+T/JT2cbDRKUXxkGf0v0qjgt4etALI/tdDK6Yhhtz2/hqvouJxxzvyld PkATKiZVVSpIUT6pcz4nskOqVB8L1+ef8kfls06Va78Vboic5Y5vtZgvxS1fWIxZ C0m9kwcvfVpBePbaaYm5mpoSuVJv/p6DE/tMdtt3H60Qgp8CPA9v+fMrq+DrVvZ6 DXAV4yUzAGTP5Qmkb4p+Ep3k08UN+O9ndlpsvz830pmE7S0aMeyu8lQKjImRiOE= =K8Si -----END PGP SIGNATURE-----