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Re: Denotation of URIs

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 10 Apr 2003 10:07:48 -0400 (EDT)
Message-Id: <20030410.100748.88940795.pfps@research.bell-labs.com>
To: Patrick.Stickler@nokia.com
Cc: www-rdf-interest@w3.org

From: <Patrick.Stickler@nokia.com>
Subject: RE: Denotation of URIs
Date: Thu, 10 Apr 2003 09:05:10 +0300

> > -----Original Message-----
> > From: ext Peter F. Patel-Schneider 
> > Sent: 09 April, 2003 16:21
> > To: Stickler Patrick (NMP/Tampere)
> > Cc: www-rdf-interest@w3.org
> > Subject: Re: Denotation of URIs
> > 
> > From: <Patrick.Stickler@nokia.com>
> > Subject: RE: Denotation of URIs
> > Date: Wed, 9 Apr 2003 09:33:31 +0300


> > HTML doesn't say that all documents that are non-valid HTML 
> > document are
> > supposed to cause a core dump; it is just silent how such 
> > documents are to
> > be treated, and many browsers do interesting things.
> Oh come now. HTML says what is or is not valid. What is or is not
> correct. And yes, most browsers exhibit the desirable characteristic
> of graceful degredation in the presence of invalid input, which is
> a desirable characteristic for any software application.
> But invalid HTML is invalid HTML and is *wrong* even if a browser
> is able to do something with it.

Huh?  What part of the HTML specification does such a browser violate?

> Ambiguous denotation of URIs is *bad* even if SW agents are able
> to still function when it exists.

> > Let's then agree that your version of the Semantic Web is 
> > only concerned
> > with situations where denotation of URI references is fixed 
> > and universal.
> I consider this to be the only reasonable version of the Semantic Web.
> Not just "my" version of the Semantic Web.

Well I disagree.

> I consider any version of a Semantic Web which supports ambiguity
> of denotation and does not consider it an error to be ill defined
> and far less useful for the reliable communication of knowledge
> between arbitrary systems on a global basis.
> > I would then support you in any effort to state that the 
> > denotation of URI
> > references is fixed and universal given this assumption.  
> Not that is *is*, but that it is *presumed* and that if in fact
> it is determined that there is ambiguity and disagreement about
> the denotation of a given URI, that that is a problem, a but, an
> error, and is bad and should be resolved -- by whatever means
> (social, legal, technical, whatever) might be useful to do so.

Why?  People get along quite fine without this.

> I've never said (and have several times reiterated this) that
> one can guaruntee that all URIs have unambiguous global denotation
> (yet you seem to continue to harp on this point).
> It is one thing to say that the SW *presumes* unambiguous global
> denotation of URIs yet another thing entirely to say that the SW
> *guaruntees* unambiguous global denotation of URIs. I assert the
> former. I have never asserted the latter. Please take note of this.

I don't see the difference here.  If (the formal machinery of) the SW
presumes that that URIs have unambiguous global denotation then tools are
going to have this presumption built into them.  They will break if the
presumption is violated.  

> > Feel free to work
> > under this assumption provided that you don't eliminate the 
> > possibility of
> > other visions of the Semantic Web that lift this assumption, 
> > which is what
> > you appear to have been attempting to do.
> Absolutely. I consider it harmful to the SW architecture to
> posit that it is in any way *acceptable* to have ambiguity in
> denotation. It is IMO *never* acceptable to have such ambiguity,
> and in any case where there is ambiguity of denotation, it will
> be harmful to the SW.

Well, this is your opinion.  I disagree.

> > > The same is true for SW agents. Ambiguity in denotation 
> > should be clearly
> > > and strongly identified as bad, wrong, detrimental, etc. yet the SW 
> > > architecture can still have machinery to help identify any occurring
> > > ambiguity and deal with it productively.
> > 
> > Drawing a line around the part of the Semantic Web that needs this
> > assumption is fine.  When working in this part of the 
> > Semantic Web it is
> > also fine to try to identify parts of the Semantic Web that 
> > don't quite
> > satisfy the assumption and try to modify them as necessary to 
> > satisfy the
> > assumption. 
> I just don't see how it is possible to reconcile the two views
> that (a) ambiguity of denotation is bad and should be avoided,
> eliminated, fixed and (b) ambiguity of denotation is fine and
> acceptable.

Well I don't need to reconcile the two views because I don't believe that
the first view is correct.

> I understand that some folks may wish to be able to explore
> various means to deal with ambiguity, and that's great, since
> I'm quite sure that we will face ambiguity of denotation of the
> SW and it will be wonderful to have effective means of dealing
> with it.

Great.  Then let me get on with it.

> > > Being able to deal with an error does not validate the 
> > error and make it OK.
> > 
> > Well, sure, if you view it as an error.  However, others may 
> > disagree, and
> > view it as a source of power.
> Well, I'd be quite surprised if a majority of folks would not
> consider it an error.

Sure, a majority of folks may want to work under this assumption.  I don't
mind this at all.  I don't even mind this being a blessed assumption for
parts of the Semantic Web.  Just don't make it be part of the formal
machinery of all of the Semantic Web.

> And I've yet to see any use case, from you or elsewhere, which
> demonstrates even marginal benefit from the existence of
> ambiguity in URI denotation.

What about my ``first lady'' example?

> I've provided several examples showing how it is detrimental
> to the SW and could easily provide many more.

Sure, denotation ambiguity is difficult to deal with in many case.  I don't
deny that.  If tools are built based on this assumption, then of course
violating it will be detrimental to their functioning.  However, limited
ambiguity can be handled, as people do it all the time.

> Anyone who wishes to argue that the SW architecture should "bless"
> ambiguity in URI denotation is going to have to back it up with
> clear and motivating evidence that it is beneficial.

Well, what is the vision of the Semantic Web?  One vision, that used to be
in the RDF documents, is that the Semantic Web, even in the form of RDF, is
supposed to be able to ``say anything about anything''.  If it is not even
possible to have divergence of denotation, then it sure won't be possible
to talk about it.

> I wait to see that evidence...
> > > Ambiguity in denotation will always be a bad thing. Always. 
> > Even if SW agents
> > > can deal with it trivially (which I doubt they will be able 
> > to do short of
> > > cognitive abilities).
> > 
> > Ambiguity in denotation is a fact of communication.  Always. 
> Have I ever said anything in any of my posts that indicated I
> think otherwise? No.

No, but you sure seem to be trying to eliminate it from communication in
the Semantic Web.

> When I say that "ambiguity in URI denotation is bad" you seem to
> be interpreting that as meaning "ambiguity in URI denotation is
> impossible". Please feel free to interpret what I say literally.

I'm not saying that you said that "ambiguity in URI denotation is
impossible".  I am, instead, saying that ambiguity is a fact of
communication, and that therefore the Semantic Web will have to deal with

> Again, being able to deal with *problems* does not validate
> those problems and make them OK.
> > SW agents
> > will eventually have to deal with it (and should be able to 
> > deal with some
> > ambiguity in denotation without full cognitive abilitities).   
> > 
> > It may be that some cognitive-like abilities are needed to 
> > deal with any
> > sort of ambiguity in denotation, but so what?  Aren't SW 
> > agents supposed to
> > have cognitive-like abilities?
> They might, but need not. And I think it will be some time before
> they will be able to handle ambiguity of URI denotation effectively,
> which is why it must be a fundamental assumption of the SW architecture
> that SW agents may *presume* that URIs have unambiguous denotation.

Then the Semantic Web will have to be replaced by something else in the
not-too-distant future.  I would be much happier if this wasn't necessary,
and that ambiguity of denotation would not be ruled out of all of the
Semantic Web.

> Any ability of SW agents for dealing with ambiguity will ultimately
> serve the purpose of identifying and correcting such ambiguity, IMO,
> not encouraging further ambiguity on the SW. It will be a long, long
> time (decades, I think) before ambiguity in URI denotation would
> become innocuous on the SW.

I don't think that ambiguity in denotation will ever be innocuous.  I think
that it will always be difficult to deal with.  That is, however, no reason
to rule it completely out.

> So I'll assert again (for the umpteenth time) just because a SW agent
> can deal with noise does not mean that noise is good or beneficial
> or correct or acceptable.

Sure, but that doesn' t mean that ambiguity in denotation should be

> Perhaps I'm misunderstanding you, but you seem opposed to the idea
> of saying that ambiguity in URI denotation is a bad thing. You seem
> to be asserting that it's OK. Feel free to correct me if I've not
> understood your position corectly.

A am opposed to saying that ambiguity in URI denotation is a bad thing, at
least for the Semantic Web in general.  I'm not opposed to you building
applications that view ambiguity in URi denotation is a  bad thing, just
don't foist that view on my applications.

> Patrick

Received on Thursday, 10 April 2003 10:08:07 UTC

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