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Re: use/mention and reification

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Wed, 23 Jan 2002 19:17:05 +0200
To: ext Frank Manola <fmanola@mitre.org>
CC: ext Graham Klyne <Graham.Klyne@mimesweeper.com>, RDF Core <w3c-rdfcore-wg@w3.org>
Message-ID: <B874BCB1.C29C%patrick.stickler@nokia.com>
On 2002-01-23 17:42, "ext Frank Manola" <fmanola@mitre.org> wrote:
> ...  
> 2.  If two people use different non-URIs, like "Superman" and "Clark
> Kent", to refer to (apparently) different things, the same thing holds.
> We know they've used different names, but we may still infer they are
> talking about the same thing if we get enough additional information.

The same "thing" in the universe, yes, but not the same RDF resource.
We wouldn't want all nodes with Superman and ClarkKent URIs to
be merged, even if we determine that they denote the same "thing".

> 3.  If two people use the same URI, like ex:Superman, to refer to
> something, we know that they *think* they are talking about the same
> thing, 

This equates to my present view, that if two or more folks
use the same URI, they are agreeing that it denotes the same
"thing", whatever that "thing" may be.

That doesn't mean that some "thing" in some universe may not
be denoted by multiple URIs, or by "constellations" of properties,
including literal encoded names, etc. Only that, insofar as that
single URI is concerned, its use represents agreement according
to what the minting authority says it denotes.

> but they may not be.  E.g.,
> a.  One person may have introduced the URI ex:Superman, and someone else
> starts using it, but the second person actually has something else in mind.

It is IMO the right of the minting authority to specify what
a URI means. And if someone else uses it incorrectly, even
if unintentionally, they shouldn't usurp ownership of the meaning
of  that URI but should either conform to the specified meaning
or use another URI. Obviously, that's a social issue (or legal
one) not a technical issue.

> b.  The two people involved have the same entity in mind, but have
> different understandings about that entity's attributes.  Consider
> ex:Clark Kent.  One person thinks Clark is identical to Superman, and
> the other (Lois) doesn't.  One person thinks Clark has Xray vision (and
> conceals it), the other thinks Clark has ordinary vision.

But both Clark Kent and Superman would presumably be denoted by
different URIs, and belief could be expressed in terms of either.

If Superman has Xray vision, and it is later determined that Superman
is the same thing as Clark Kent, then one many infer that Clark
Kent also has Xray vision. But even if the are the same thing in
the universe, they are different resources in RDF space (if
denoted by different URIs).

> I don't see how we can reasonably expect URIs to be used only in
> situations where everyone on the Web has a full understanding and
> agreement on all the attributes of the thing the URI names before they
> start using it. 

There need not be agreement about all attributes of a thing, only
about what thing a given RDF resource denotes.

> What we do get using URIs is the extra bit of
> information suggested by #3 above:  that when people use the same URI,
> they at least *think* they are talking about the same thing.

Of course one cannot prevent misunderstanding, or even disagreement,
on the SW, but agreeing on the same URI to denote the same thing
seems a reasonable basis to conduct discussions, and we can hope
that knowledge expressed in terms of RDF and URIs will be less
ambiguous than knowledge expressed in natural language ;-)

> That's 
> useful information!  You can merge their different beliefs via the URI
> in their various RDF graphs, and then possibly detect inconsistencies,
> which could then lead to better agreement after some dialog.

And that's the beauty of RDF. Users who use URIs are not agreeing
about all of the attributes of a thing. Only the denotation of
that thing by which they describe the attributes of that thing
that *they* know about, or by which they obtain attributes known
about that thing by others.

Then knowledge is pooled, including knowledge about
the equivalences of RDF resources which denote the same things,
and viola, we get a synergetic merging of knowledge that may
be far more than the simple sum of its parts.

And if that merged body of knowledge contains contraditions
(which it likely will) then as you suggest, these could serve
to clarify common understanding (or misunderstanding).

Of course, there can still be disagreement even if everyone is
in complete agreement about what thing a given URI denotes.

> Another way of putting this is to ask if not allowing URI labeled
> resources to have different denotations means that different people
> can't attribute contradictory things to the same URI labeled resource?

I think that folks can attribute contradictory things about the
same URI labeled resource without requiring that it denote more
than one thing.

I consider the N-to-1 mapping of RDF resource(s) to a thing as
fundamental and essential as the N-to-1 mapping of lexical form(s)
to a datatype value. If we allow N-to-N mappings then we lose all
clarity in our statements. I.e. if a given URI can denote more than
one thing in a universe, then we'd never know precisely what it is
anyone is talking about.

This leads us right into the heart of the URI/URL/URN/URP debate
where some folks think that e.g. 'mailto:john.doe@abc.com'
can denote both an email URL and a person, or 'http://www.abc.com' can
be used to denote both a web page URL and the company ABC Inc.
This introduces an N-to-N mapping from the set of RDF resources
to the set of things in the (or a) universe such that if
one says

   <http://www.abc.com> <dc:creator> "John Doe" .

one cannot know if John Doe is the creator of the web page or
of the company.

The "contemporary view" seems to take the view that "a URI is
whatever it is used for" similar to Perl scalars, yet this
introduces this N-to-N mapping from resources to things that
makes inference about knowledge untenable.

I keep thinking that surely there must exist some axiom
or "law" in KR or AI that forbids such N-to-N mappings
in a KR system (my ignorance shames me).


> --Frank



> Patrick Stickler wrote:
>> On 2002-01-23 14:10, "ext Graham Klyne" <Graham.Klyne@MIMEsweeper.com>
>> wrote:
>>> At 01:46 PM 1/23/02 +0200, Patrick Stickler wrote:
>>>>> OTOH, without allowing
>>>>> that it seems that URI-refs don't offer anything that label strings like
>>>>> "Lois" and "Jimmy" don't also offer.  The point of my comment was to
>>>>> suggest that matters relating to personal belief of identity shouldn't
>>>>> really be expressed in terms of URIs.
>>>> But your examples are not about belief of identity, they
>>>> are about belief of properties of entities which are given
>>>> identity.
>>>> I.e., you weren't saying that <person:Lois> <ex:accepts> that
>>>> "Superman" <ex:is> <person:Superman>.
>>> My take on this scenario was that Lois' non-belief that Clark Kent is
>>> strong is rather bound up with her belief about the identity of the person
>>> she knows as "Clark Kent";  i.e. that she does not recognize him as also
>>> being the person she knows as "Superman".
>>> I think some alternative formulation of this scenario could lead to a
>>> situation in which Clark Kent and Superman have universally accepted
>>> denotations, and URIs may then be appropriate.  In this case, I think that
>>> it's not possible that they denote exactly the same thing;  e.g. Clark Kent
>>> denotes a person X wearing a suit and glasses;  Superman denotes the same
>>> person X wearing a natty blue-and-red number.  In this formulation, using
>>> URIs seems less troublesome.
>> I see your point. Though I'm not sure that the Dan's do. Talking about
>> some person X named "Superman" (who might later be deemed to equate to
>> <person:Superman>) is not the same as using "person:Superman" rather
>> than <person:Superman> to avoid instantiating a URI labeled resource
>> node in the RDF graph.
>> Eh?
>> Patrick
>> --
>> Patrick Stickler              Phone: +358 50 483 9453
>> Senior Research Scientist     Fax:   +358 7180 35409
>> Nokia Research Center         Email: patrick.stickler@nokia.com

Patrick Stickler              Phone: +358 50 483 9453
Senior Research Scientist     Fax:   +358 7180 35409
Nokia Research Center         Email: patrick.stickler@nokia.com
Received on Wednesday, 23 January 2002 13:38:53 UTC

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