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RE: Denotation of URIs

From: <Patrick.Stickler@nokia.com>
Date: Thu, 10 Apr 2003 09:05:10 +0300
Message-ID: <A03E60B17132A84F9B4BB5EEDE57957B5FBB7B@trebe006.europe.nokia.com>
To: <pfps@research.bell-labs.com>
Cc: <www-rdf-interest@w3.org>

> -----Original Message-----
> From: ext Peter F. Patel-Schneider 
> [mailto:pfps@research.bell-labs.com]
> 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
> [...]
> > > Ambiguity is difficult to deal with, agreed, and it is 
> reasonable to try to
> > > reduce it to the extent possible.  However, mandating 
> that certain kinds of
> > > ambiguity are not permissable, just means that one cannot 
> deal with
> > > situations where these kinds of ambiguity actually exist.  
> > 
> > I disagree. One can mandate all kinds of constraints yet 
> still provide
> > machinery for dealing with violations of those constraints. 
> A case in
> > point are HTML browsers. The HTML specs are very clear 
> about what is or
> > is not a valid HTML instance, yet browsers are quite 
> capable of dealing
> > with errors in HTML instances.
> 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.

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.

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 
> denoation 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.

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.

> 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.

> > 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

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.

> > 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.

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.

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

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.

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.

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.

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.

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.

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.

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.


Patrick Stickler, Nokia/Finland, (+358 40) 801 9690, patrick.stickler@nokia.com
Received on Thursday, 10 April 2003 02:05:14 UTC

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