Pat Hayes wrote:
>> 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.
>
> Yes, indeed. And in fact RDF provides an explicit vocabulary for this,
> called the reification vocabulary. (01)
No, this is not quite what I was after here. The reification vocabulary
allows one to talk about triples, yet there is no way, in RDF(S) to
connect a triple to a triple about it, otherwise than by explaining it
informally in words. (In the same way as there is no way to connect a
URI to the dog it denotes, say. But since URIs themselves are character
strings, they are exceptional in that they may appear directly in a
triple, unlike dogs.) (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
>
> 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. (03)
I acknowledge your efforts in the case of the RDF semantics document,
with some minor comments, if you're interested. But the RDF primer, for
example, does need some polishing. (04)
No offense intended ;) (05)
>> , 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.)
>
> 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)
In the docs, you differentiate between a triple (in the abstract) and a
particular realization of it, in some physical document. This is
perhaps what you mean here, but this still does not seem to me to be the
distinction between a proposition and an expression. The abstract
triple is a formula (an abstract syntactic form), but not a proposition. (07)
(But I am not willing to discuss the issue of the ontological status of
propositions in depth.) (08)
>
>> 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.
>
> 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. (09)
But a literal may be an object of a triple, and thus a triple may
explicitly state that a URI has a URI as a referent. (010)
>>
>> 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.
>
> 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. (011)
No, you're wrong here. Since the URI in the subject position is treated
as a reference and not as an entity, you can't make a statement such as
'u1 denotes u2'. You need to make a statement such as 'u1 is u2', where
the first URI is dereferenced, and the other is not, which amounts to
say that the referent of u1 *is* (and *not* refers to) u2 (and *not* the
referent of u2). (012)
You can of course use in the predicate position a URI that denotes the
relation of reference, but then 'u1 refers to u2' does not mean that u1
is a URI used to refer to u2, but rather that u1 refers to something
(presumably a URI) that refers to u2. (013)
vQ (014)
>
> Pat Hayes
>
>>
>> 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
>>
>>
>>
>> --
>> Wacek Kusnierczyk
>>
>> ------------------------------------------------------
>> Department of Information and Computer Science (IDI)
>> Norwegian University of Science and Technology (NTNU)
>> Sem Saelandsv. 7-9
>> 7027 Trondheim
>> Norway
>>
>> tel. 0047 73591875
>> fax 0047 73594466
>> ------------------------------------------------------
>>
>>
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>
> (015)
--
Wacek Kusnierczyk (016)
------------------------------------------------------
Department of Information and Computer Science (IDI)
Norwegian University of Science and Technology (NTNU)
Sem Saelandsv. 7-9
7027 Trondheim
Norway (017)
tel. 0047 73591875
fax 0047 73594466
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