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
>>>> 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.
>>
>> 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.
>
> 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.
> ... (01)
Still holding... Yes, this is clear from the (suggested) semantics of
the reification vocabulary. But I were far from even mentioning
reification. (02)
>
>>
>>>>
>>>> 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). (03)
What I was saying is that you the triple (04)
u1 u2 u3 (05)
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. (06)
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. (07)
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 self-referential: (08)
protocol://domain/some-name protocol://domain/identity-relation
"protocol://domain/some-name" (09)
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. (010)
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. (011)
There is no quotation, but since URIs are strings, a string
indiscernible from a URI is in effect identical with the URI. (012)
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. (013)
vQ (014)
>
> Pat
>
>> , which amounts to say that the referent of u1 *is* (and *not* refers
>> to) u2 (and *not* the referent of u2).
>>
>> 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.
>>
>>
>> vQ
>>
>>>
>>> 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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>>>
>>>
>>
>> --
>> 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
>> ------------------------------------------------------
>
> (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
------------------------------------------------------ (018)
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