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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: Mon, 16 Apr 2007 14:27:52 -0500
Message-id: <p06230905c2497cdaafc4@[]>
>Pat Hayes wrote:
>>>>>, 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
>>>>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.
>>>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 
>>No, thats not what I meant. I was referring to the suggested truth 
>>conditions for reified triples, in which the (take a deep breath) 
>>object of the subject property is the referent of the subject URI 
>>of the original triple, not the subject URI itself.
>Still holding...  Yes, this is clear from the (suggested) semantics 
>of the reification vocabulary.  But I were far from even mentioning 
>>>>>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.
>>>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 can if you have a name for u1. Of course you cannot use the URI 
>>to name itself, but you had already noted that point (on which we 
>>agree, of course).
>What I was saying is that you the triple
>u1 u2 u3
>does not *necessarily* say anything about the URI u1;  it may, 
>though, if u1 is self-referential (and why couldn't it be?)  That 
>is, there is nothing in the RDF semantics that would forbid a URI to 
>name itself.
>The *form* of the statement in RDF would be a triple like
>>u3 denotes u2 .
>>where 'u3' is the URI used to refer to the URI 'u1'. My point - and 
>>I think we are in violent agreement here - is that the relationship 
>>of naming, which has to hold between u3 and u1 in order for this to 
>>be any use, is something outside the RDF model. But this should not 
>>be surprising: RDF is very simple, and even its own reification 
>>vocabulary needs some kind of external machinery to connect the 
>>subject URI of the reification graph to the original triple which 
>>the reification is supposed to describe.
>>>  You need to make a statement such as 'u1 is u2', where the first 
>>>URI is dereferenced, and the other is not
>>You can't do it that way (in legal RDF) as the RDF spec requires 
>>all URIs to be dereferenced.
>I do can, since a URI is just a sequence of characters organized 
>according to specific syntactic rules (this is a syntactically 
>modified but semantically rather accurate definition of URI).  Thus, 
>the following triple has the effect of asserting that a URI is 
>protocol://domain/some-name protocol://domain/identity-relation 
>where "protocol://domain/some-name" is a string literal which 
>happens to be a valid URI, the same as used in the subject (of the 
>triple).  Thus, the referent of the URI protocol://domain/some-name 
>is in the relation of identity (say) with the string enclosed in 
>double quotes (which do not belong to the string!) -- that is the 
>URI denotes itself.
>There is no quotation in RDF. Perhaps we should
>>have put it in, but its too late now. In fact, reification seems to 
>>have died and not be widely mourned, so maybe this omission is not 
>>in fact of great importance.
>There is no quotation, but since URIs are strings, a string 
>indiscernible from a URI is in effect identical with the URI.
>But even if this fails, for some reason, you can use a URI to refer 
>to itself by just intending to do so, as you would intend to use a 
>URI to denote a dog, say.
>vQ    (01)

Ahhh, I see now what you have been saying. Sorry if I was slow. Yes, 
you are right, in princiople we could have slef-referntial URIs, and 
their characteristic hallmark woul dbe that they were equal to their 
own quotations. (It would be better to use a typed literal with 
xsd:anyURI, which is unambiguously understood to refer to URI, even 
if URIs are considered different from strings (which they are by some 
people; giving    (02)

protocol://domain/some-name protocol://domain/identity-relation 
.    (03)

But this cannot be a general solution, since most triples contain 
URIs whose intended interpretation is not themselves, but something 
that was the topic of the representation containing the triple in the 
first place. Making them refer to themselves will change their 
meaning, and the meaning of the ontologies that contain them. So what 
we need as a general device is a way to create a new URI (<u3> in my 
message) which refers to a given URI, i.e. in effect a quoting 
mechanism. Such a thing could of course be set up, by defining a 
'uri-quoting' name space and forming the new u3 URI out of it and the 
old u1 URI in some systematic way.    (04)

Im afraid I have lost track of what this purpose of this thread is :-)    (05)

Pat    (06)

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