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Re: [ontolog-forum] OWL and lack of identifiers

To: Waclaw Kusnierczyk <Waclaw.Marcin.Kusnierczyk@xxxxxxxxxxx>
Cc: Ontolog Forum <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Pat Hayes <phayes@xxxxxxx>
Date: Sun, 15 Apr 2007 21:36:57 -0500
Message-id: <p0623090ac2488d857382@[]>
>Re: Peter F. Brown's post on URIs in OWL (Thu, 12 Apr 2007 21:44:59)
>Peter writes:
>My comment on the webmeeting chat line was slightly facetious when I
>stated that: ³If OWL added identifiers as you suggest, that would break
>Tim Berner-Lee's underlying model for Web architecture², but I think
>that you hit the heart of one of the problems that I have with OWL and
>with the W3C axioms about information modelling: that is the failure to
>distinguish between:
>- a URL as an identifier of something, (in the terms you refer to in
>slide 34 [1]); and
>- a URL that is the ³something² (a resource)
>and the fact that you can¹t actually make any assertion about a URL
>being considered as an identifier.
>It appears to me that it is possible to distinguish, in RDF(S), between
>the use and mention of a URI.    (01)

Yes, indeed. And in fact RDF provides an explicit 
vocabulary for this, called the reification 
vocabulary.    (02)

>Since any RDF(S) expression is reducible to an eqivalent set of triples,
>I will use the triple notation to illustrate the discussion.
>Let u1 and u2 be any arbitrary URIs.  Then the triple:
><u1> <u2> <u1>
>has the intended meaning that what u1 is intended to identify is in the
>relation which u2 is intended to identify with what u1 is intended to
>identify.  (Let us agree, for simplicity, to use this form: "the triple
>means that the referent of u1 is in the relation identified by u2 with
>the referent of u1".)
>So u1 is neither the subject nor the object of the proposition expressed
>with the triple.  (RDF(S) docs are somewhat messy in their use of the
>terminology here    (03)

I protest :-) We spent a great deal of time and 
care to ensure that the RDF(S) documents were 
consistent in their use of terminology.    (04)

>, but we could say that u1 is the subject of the triple,
>etc., but then we need to be clear in distinguishing the triple as an
>expression and the proposition it expresses, of which u1 is not the
>subject.)    (05)

Right. The notion of proposition does not really 
get used in RDF. Although if you check the 
recommended semantics for reified triples, the 
reification is interpreted as a proposition 
rather than a syntactic expression.    (06)

>But you can make assertions about u1 itself.  The subject of an RDF(S)
>triple is always treated as a URI.  So to make an assertion about a URI
>(rather than about its referent), you need a URI identifying your URI,
>and you're done.    (07)

You need some way to refer to the URI (actually 
URI reference), but it need not be a URI. The 
most natural name to use would be a typed literal 
with the datatype xsd:anyURI . Unfortunately, RDF 
does not allow a literal to be a subject, which 
makes for some awkwardness.    (08)

>Note that an RDF(S) triple may have a literal rather than an identifier
>as the object.  Using this form, you can explicitly connect a URI to
>another URI, so that the latter is the referent of the former.  Although
>the core RDF(S) vocabulary does not provide you with a URI for the
>relation of identity, there is nothing in the way to create one.    (09)

But this relation isn't identity, its more like 
'refers to' (actually its inverse). BUt I agree, 
one can add this property. It can't be described 
in OWL or even in normal FOL, so one needs some 
external machinery or extension in any case.    (010)

Pat Hayes    (011)

>Consider this triple:
><u1> <u2> u1^<uri>
>where <uri> is a URI for the xml-schema URI datatype.
>What the triple says is that the referent of u1 is in the relation
>identified by u2 with the uri u1 (not with the referent of u1!).
>If u2 identifies the relation of identity, then the triple effectively
>asserts that u1 is self-referential, and any other triple with u1 as the
>subject (the subject of the triple) is an assertion about u1 itself (u1
>as the subject of the proposition).
>I find this far from unclear or confused;  the confusion arises, as far
>as I can see, from improper interpretation of the syntax and semantics
>of RDF(S).
>Conclusion:  if you want to make assertions about URIs rather than about
>their referents, use URIs that identify your URIs.
>(The original post was about OWL and URLs, but I hope that we can agree
>that URLs are specialized URIs, and that OWL is an extension of RDF.)
>Wacek Kusnierczyk
>Department of Information and Computer Science (IDI)
>Norwegian University of Science and Technology (NTNU)
>Sem Saelandsv. 7-9
>7027 Trondheim
>tel.   0047 73591875
>fax    0047 73594466
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