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

To: Ontolog Forum <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Waclaw Kusnierczyk <Waclaw.Marcin.Kusnierczyk@xxxxxxxxxxx>
Date: Sun, 15 Apr 2007 19:22:01 +0200
Message-id: <46225F39.80400@xxxxxxxxxxx>
Re: Peter F. Brown's post on URIs in OWL (Thu, 12 Apr 2007 21:44:59)    (01)

Peter writes:    (02)

My comment on the webmeeting chat line was slightly facetious when I
stated that: “If OWL added identifiers as you suggest, that would break
Tim Berner-Lee's underlying model for Web architecture”, 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:    (03)

- a URL as an identifier of something, (in the terms you refer to in
slide 34 [1]); and
- a URL that is the “something” (a resource)    (04)

and the fact that you can’t actually make any assertion about a URL
being considered as an identifier.
"    (05)

It appears to me that it is possible to distinguish, in RDF(S), between
the use and mention of a URI.    (06)

Since any RDF(S) expression is reducible to an eqivalent set of triples,
I will use the triple notation to illustrate the discussion.    (07)

Let u1 and u2 be any arbitrary URIs.  Then the triple:    (08)

<u1> <u2> <u1>    (09)

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".)    (010)

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, 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.)    (011)

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.    (012)

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.    (013)

Consider this triple:    (014)

<u1> <u2> u1^<uri>    (015)

where <uri> is a URI for the xml-schema URI datatype.    (016)

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).    (017)

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).    (018)

Conclusion:  if you want to make assertions about URIs rather than about
their referents, use URIs that identify your URIs.    (019)

(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.)    (020)

Wacek    (021)

Wacek Kusnierczyk    (022)

Department of Information and Computer Science (IDI)
Norwegian University of Science and Technology (NTNU)
Sem Saelandsv. 7-9
7027 Trondheim
Norway    (023)

tel.   0047 73591875
fax    0047 73594466
------------------------------------------------------    (024)

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