W3C home > Mailing lists > Public > uri@w3.org > April 2003

RE: Resources and URIs

From: <Patrick.Stickler@nokia.com>
Date: Tue, 29 Apr 2003 11:41:59 +0300
Message-ID: <A03E60B17132A84F9B4BB5EEDE57957B5FBBA7@trebe006.europe.nokia.com>
To: <phayes@ai.uwf.edu>
Cc: <gk@ninebynine.org>, <uri@w3.org>

> -----Original Message-----
> From: ext pat hayes [mailto:phayes@ai.uwf.edu]
> Sent: 29 April, 2003 01:51
> To: Stickler Patrick (NMP/Tampere)
> Cc: phayes@ai.uwf.edu; gk@ninebynine.org; uri@w3.org
> Subject: RE: Resources and URIs
> >  > You tell me, how exactly does one 'figure out' what the URI
> >>  http://www.w3.org/2001/XMLSchema is "bound" to? Even a 
> human being?
> >>  Try looking up "namespace" in 
> http://www.m-w.com/netdict.htm and see
> >>  what you get, for a start.
> >
> >This is what I have been working on URIQA to do. Given a URI, you can
> >ask a SW-enabled server (either the same server that 
> provides representations
> >via HTTP GET, or another server, via an explicit URIQA 
> portal) to tell
> >you about the resource, and the knowledge about the resource 
> may (hopefully,
> >will) enable you to clarify what the URI denotes, what that resource
> >actually is. At the very least, it should tell you a bit 
> about the nature
> >of the resource denoted by the URI.
> OK, that sounds great. But look at what you said: it will tell you 
> something about the resource.  Maybe that will be enough for you to 
> identify what it denotes; maybe not (Im not *entirely* sure what 
> Nokia means by 'wireless terminal', but Im confident I could find out 
> if I needed to); but in any case, that isn't the same as binding the 
> URI to a unique referent. 

Absolutely not. It's not meant to be. Ultimately, the URI to resource
mapping will remain outside the machinery of the SW and any sleuthing
a SW agent might do based on descriptions of the resource in order
to clarify that denotation based on similar descriptions of other
resources denoted by other URIs will always remain conjecture, for
the SW agent.

> Its just requesting, and getting, 
> information using the URI, just like all other uses of the URI: a 
> particularly pointed kind of information, maybe, which is intended to 
> fix the referent well enough to be useful, but still, information. 
> And if anything, this URIQA seems to bear out the MT idea rather than 
> the RFC 2396 fantasy, since if there was only one possible referent 
> of the URI - only one way to interpret it - then I wouldn't *need* to 
> ask URIQA about it.

Well, I never intended to suggest that any knowledge provided by
a URIQA server would fix the interpretation of a URI -- rather I
was trying to show that such knowledge could clarify e.g. whether
the resource denoted was a web page, an image, a person, etc.
and (perhaps a bit too implicitly) presuming that the information
was at some level being interpreted by a human being.

I.e. given two people arguing over whether some URI denotes a web
page or a person, they could request an authoritative description
from the web authority and see if the resource is asserted to
belong to some class or has some other properties, which would
resolve that debate.

> >
> >E.g. what does the following URI denote?
> >
> >   http://forum.nokia.com/product/terminal/N6310r100
> >
> >(at the moment, there are no representations, so GET returns 404)
> >
> >You can't (and no fair peeking inside the URI to guess from any
> >possible mnemonic interpretations of any substrings therein, for
> >all you know it could be a "terminal (failed) product" ;-)
> >
> >But you can ask the Nokia Semantic Web server for a description
> >of the resource
> >
> > 
> >http://sw.nokia.com/swe/URIQA?uri=http://forum.nokia.com/prod
> uct/terminal/N6310r100
> >
> >and you find out that it is a Wireless Terminal, along with 
> other information
> >such as its UAProf profile.
> Cute!
> >
> >Soon, you will be able to ask the authoritative server 
> >forum.nokia.com directly
> >about its resources and not have to use a proxy URIQA agent 
> (though this also
> >demonstrates how both authoritative knowledge from the same server
> >that provides representations and third party knowledge from other
> >servers with URIQA portals can work in unison to serve 
> knowledge about
> >resources on the SW.
> >
> >(BTW, this is a work in progress, so don't be surprised if it pukes
> >now and then, it's not yet ready for prime time... ;-)
> >
> >>  Yes, but that is true for all URIrefs, everywhere.  
> Anyone else can
> >>  assert that my webpage denotes the Queen. 
> >
> >How would they do that? *IN* RDF I mean.
> Well, yes; you can't ground a URIref to a denotation *just* by using 
> RDF. But you might say enough using RDF to link it tightly enough to 
> other grounded URIrefs, eg by talking about being a reigning monarch 
> of Britain and so on, to force the reference. But my point was 
> different here: I was just making the point that anyone else can use 
> my URIrefs to say stuff that I wouldn't want to say. I don't have any 
> control over what is said USING my URIs.

Sure. I've tried to maintain a clear distinction between the denotation
of a URI and statements made about the denoted resource using that URI.
This is a very important distinction.

And of course, it is quite true that, just as others can make statements
about some resource using a URI you minted (where you all agree about
the denotation) likewise others can mis-use a URI you minted to denote
something entirely different. And both cases are such that must be
resolved using social (even legal) means.

But if I, as a completely neutral and ignorant third party, recieve
RDF statements from all of you, I am going to presume that you all
agree at least about the denotation of the URIs you are using.

How I deal with contradictory statements, or prioritizing of sources,
is another issue entirely.

> >
> >You'd at least have to have some other URI that denotes the Queen,
> >and say something like
> >
> >    x:PatsWebPage owl:sameAs y:TheQueen .
> >
> >But then, how does one define the denotation of y:TheQueen?
> >
> >At some point, we have to accept that the machinery for 
> actually defining
> >the URI to resource mapping is outside the scope of the SW machinery.
> Well, OK, though I think a lot of it is actually there in the Web 
> machinery more generally. That is, the Web is actually a mix of real 
> grounded references and other nongrounded usages, and there are 
> representations mixed up in there as well. Its wonderfully 
> complicated, and it works.

It works as the Web, but not as the SW. See my other posts about this.

What constitute "grounded references" on the present Web are sources
which require human interpretation. At some point, for SW agents, 
you've got to decide where to draw the line and what your minimal
set of atomic primitives will be.

If you are able to devise some kind of ultimate semantic vocabulary
by which you can unambiguously define the meaning of all other terms
(URIs) used, I will be very very impressed, but even then, the terms
of that ultimate vocabulary will still have to be taken as atomic
primitives that SW agents will not be able to inspect or test to
any degree.

> >You've got to have primitives at some foundational layer, 
> and for RDF,
> >those primitives are the URIs. How those URIs are mapped to resources
> >is based on processes ouside of RDF. No?
> >
> >>  But these URIrefs 'belong'
> >>  to the server, so any assertions made about them from 
> another source
> >>  are not warranted by the server.  I don't see any problems arising
> >>  here that aren't endemic to the entire Web (and so are 
> unlikely to be
> >>  real problems, in fact, since the Web seems to work quite well.)
> >
> >Insofar as http: and similar URIs are concerned, which include a
> >web authority component, I fully agree. The web authority component
> >serves as a basis for distinguishing authoritative knowledge about
> >a resource from third party knowledge.
> >
> >(Whether a SW agent ranks any third party knowledge the same as or
> >even higher than the authoritative knowledge is an entirely different
> >issue)
> >
> >>  ...If you like, think of it as the question, is it kosher
> >>  to Skolemize on the Web? That is, can I make up a new URI and say
> >>  that it denotes something, just on the basis of knowing that
> >>  something *exists*, and not knowing anything else about it? If so,
> >>  how do I "bind" this thing - about which I know virtually 
> nothing -
> >  > to my URI, or make my URI "identify" it?  Particularly if I can't
> >>  identify it and I have nothing to bind it with.
> >
> >This is why I consider the binding machinery to be below the level
> >of primitives for RDF and the SW. SW agents need not be 
> concerned with
> >how a URI is bound or mapped to a thing in the real world, only that
> >when it encounters a given URI, it can presume it consistently refers
> >to the same thing (not that it always *will*, after all, 
> shit happens,
> >but that it can -- and should -- *presume* that it does).
> OK, but this is where I don't follow the reasoning. Apart from my 
> gripes to Graham about not fantasizing, WHY should one presume that 
> it does? 

Because otherwise, there is no hope of reliable global interchange
of knowledge. Period.

> Seems to me that this assumption  doesn't add any 
> functionality, but gets in the way of a coherent semantics. For 
> example, consider an agent which knows zilch about what a URI 
> denotes, but which then consults UIRQA and then knows a lot more 
> about it. How can we characterize what it has learned? If URIs *have* 
> to denote what they "really" denote, in all interpretations, then we 
> can't say that the agent has learnt anything expressible as a 
> sentence or proposition; yet it seems to me that is exactly what it 
> has done.

It has not necessarily learned what the actual resource is that
is denoted (but in fact, so what, since it's really humans that
care about and interpret the actual URI to resource mappings, not
the SW agents). But it may have learned something that, when combined
with other knowlege allows it to infer something new, and inform
a human user about that new knowledge, who will then interpret the
URI in terms of its mapping to some actual resource and evaluate
that knowledge against the real world.

A SW agent does not *need* to know about the actual denotation. Only
that it should presume that a URI only ever denotes one thing.

> Or suppose one of DanBri's engines does a bit of 
> owl:fuctionalProperty reasoning and discovers that PatHayes.person = 
> TheOneWithSSNumber.xxx-xx-xxx . What has it learned? According to the 
> unique-global-refernce thesis, the equality was *already true*, 
> right? It *had* to be true, in all possible interpretations. So it 
> already knew that, but it just didn't know that it knew it?

But it *has* learned something new, and important.

Before doing that reasoning, all it knew was that PatHayes.person
denotes a specific thing, and that TheOneWithSSNumber.xxx--xx-xxx
denotes a specific thing, but it did *not* know that those two
things denoted by those two URIs were in fact the *same* thing.

Now it knows that those two URIs both denote the same thing, and
that all statements made using either URI apply to the same thing.

Now *what* that thing is IN THE REAL WORLD, the SW agent doesn't 
know -- and doesn't *need* to know. It simply knows how to refer
to it, and what is asserted about it, and that, now, it can be
refered to using two URIs rather than just one.

> >  > >So I think there are two questions:
> >>  >
> >>  >(1) what is a resource?
> >>  >(2) does a URI identifiy a single particular resource?
> >>  >
> >>  >I think the answer to (2) is "yes" by my understanding 
> of URIs (e.g.
> >>  >RFC2396 section 1.1:  "An identifier is an object that 
> can act as a
> >>  >reference to *something* [that has identity]."  Even if 
> you ignore
> >>  >the problematic words [that has identity] (I think 
> they're redundant
> >>  >here), I think the words still say that the identifier 
> refers to a
> >>  >single entity:  "something" is singular.
> >
> >Graham, do you mean here that, at least by design, URIs should not 
> >be overloaded
> >to denote more than one thing?
> >
> >>  I still want to know what it means for something to be 
> "identified".
> >>  It sounds like you are saying that it means that there is a single
> >>  thing - an actual thing, not a representation of a thing 
> - which the
> >>  URI has to denote.
> >
> >I don't know if that's what Graham meant, but I certainly think so.
> >
> >A representation of a thing is also a thing. And a thing and
> >its representation must both have distinct URIs if we are to talk
> >about both of them unambiguously.
> OK, but that's another point.
> >
> >One should not expect a given URI to denote both a thing and a
> >representation of that thing,
> Right, I want meaning to get into that.
> >though this is a common error on the
> >Web as folks tend to relate what they GET from a server with the
> >thing denoted by the URI used with GET -- where in fact, what is
> >actually denoted by the URI is often, if not usually, not the same
> >resource returned by GET. Clearly, there is a common 
> disconnect between
> >perception and architecture.
> >
> >>  That is admittedly clear, but it has the
> >>  disadvantage of being an impossible fantasy.  If true, it 
> would mean
> >>  that URIs had magical properties.
> >>
> >>  This 'unique referent' claim, if taken seriously, is an incredibly
> >>  strong claim. It seems to be predicated on an assumption about the
> >>  Web which is false of all other known representational 
> and linguistic
> >>  schemes, that names are 'true names' which *inherently*, in their
> >>  very nature, identify a single thing in all possible 
> interpretations;
> >
> >The RDF graph merge function clearly IMO reflects this assumption.
> >
> >If nodes in an RDF graph represent resources, and two nodes from
> >different graphs are merged to a single node because they have the
> >same URI, then that reflects the assumption that they denote the
> >same thing.
> The same thing IN EACH INTERPRETATION.  Not the same thing in ALL 
> interpretations. The former just means that we agree to use the name 
> in the same way. The second means that the name *cannot possibly* be 
> used in any other way.

But to merge two graphs is to IMO assert that those two graphs
are valid in the SAME interpretation -- since all nodes having
identitical URIs will be merged.

OK, if folks want to have separate, isolated SWs where they maintain
their own interpretations that differ from other folks interpretations,
fine, but if we are talking about the global interchange of knowledge,
then that reflects the presumption that all interchanged graphs will
share a common interpretation!


If not, then we're all crazy to think the SW will succeed.

We're talking about a *global* SW, not just a bunch of separate,
isolated KM systems doing their own thing.

Let the RDF MT say that there can be multiple interpretations. Fine.
But make the SW Architecture specifications say that there is expected

Otherwise, there is simply no point. 

The Web has succeeded and scaled because is provided for *ARBITRARY*
yet *CONSISTENT* interaction between disparate systems based on
the explicitly defined consistent behavior of those systems.

To fail to specify the presumption/expectation of a single global
interpretation for all URIs for the SW will result in the failure
of the SW to emulate that key source of success of the Web.

SW agents should be able to interchange knowledge with any other
arbitrary SW agent anywhere on the SW and presume that there is
a shared interpretation of all URIs. Period. That is critical and
paramount to the success of a *global* SW.

Failure to embrace and formalize such a presumption/expectation
will doom the SW to fragmentation and failure.

> >My understanding of (one of) the key points of the SW is to have
> >a global knowledge base whereby arbitrary agents can interchange
> >knowledge and reason about the same things *consistently*, insofar
> >as the knowledge is concerned. I.e., that URIs would always have
> >a consistent interpretation, globally.
> >
> >If that is not the case, then I don't see the point of the SW.
> >What we end up with is simply many closed systems that simply use
> >the same infrastructure, but cannot reliably interchange knowledge,
> >since there is not even the *presumption* that there would be
> >consistent interpretation of URI denotations.
> NO no no.  Not at all.  What makes intercommunication possible is 
> indeed a presumption that what you mean by some URI is the same as 
> what I mean by it. Agreed: but that is NOT the same as saying that we 
> both have a unique thing that we must be referring to, or that there 
> is a unique thing 'bound to' the URI in all possible interpretations. 

I think I agree. There can be more than one interpretation. But the
SW presumes there is only one, and that everyone shares it. And to 
hell with all other intepretations (even if the MT provides for them).

> Think of almost any example. I'll tell you something about someone 
> you have never met: say, that my son is married. Now you know 
> something about my son.  But you don't know enough about him to pick 
> him out from all the other people in the world; my description of him 
> isn't enough to pin down one thing, the same thing in all 
> interpretations of what you know. (As it might be for your knowledge 
> of me, arguably, since you know so much more about me.) I can tell 
> you more about my son, we can still have a sensible conversation in 
> which you learn more information. If I were to assign him a URI, we 
> could use that to convey this information; yet, since I know more 
> about him than you do, there are going to be interpretations of what 
> you know in which that URI would denote things that I would recognize 
> as not referring to my son. None of this requires a single global 
> designation for the URI, or for my son to be 'bound' to the URI.

I don't disagree at all. But if you mint a URI to denote your son,
and I decide to use it to denote your wife, and make statements
about your wife using that URI, I guaruntee that someone, somewhere
is going to get very confused.

> >  > URIs, according to this, are names with LOGICALLY NECESSARY
> >>  denotations.  No other names are like that, in any human or
> >>  artificial language or naming scheme ever devised, except maybe
> >>  numerals (and even then not if you want to stay computable.). 
> >
> >Firstly, please let's leave natural language out of this discussion.
> >I don't think it is productive to compare RDF with natural
> >languages.
> I wasn't intending to do that: my point was that this issue (about 
> global interpretations) isn't true even for existing languages used 
> for communication, so denying it isn't an attack on the semantic 
> integrity of the Web. If the world in general gets by, the Web can 
> probably manage as well.

The Web is interpreted by humans, who are reasonably good with
dealing with overloading of denotation. The SW will be interpreted
by software agents, which are notoriously bad at dealing with
overloading of denotation.

Apples and oranges.

> >One of the purposes of formal languages is to escape
> >the many varied quagmires of ambiguity and imprecision inherent
> >in natural language. The comparison is not useful. Just becuause
> >natural language has certain inherent characteristics does not
> >mean that RDF or the SW must also embody them.
> >
> >Secondly, is there any reason why URIs should not be innovative?
> >Why should any other artificial language have formerly employed
> >such a naming scheme in order for the SW to employ URIs in
> >that manner. Is there some inherent pitfall or fallacy in doing so?
> The point Im trying to make is not that there is anything wrong with 
> doing anything.  It's that the way it is being TALKED ABOUT that 
> doesn't make sense.
> >Surely progress is all about doing something new and different.
> >If we are restricted only to what has been done before, then how
> >do we progress to anything better?
> Well, lets try to do anti-gravity while we are at it. BUt more 
> seriously, I don't think there are many really new uses of naming and 
> reference on the Web, in fact: what makes the Web new is that the 
> technology puts new kinds of conceptual strain on the existing 
> techniques.
> >  > Even
> >>  if you were to physically attach the names to their intended
> >>  referents, like name badges worn by people at a symposium, there
> >>  would still be some  ambiguity: does it denote the person at that
> >>  moment, the person considered as a citizen, the person's body, the
> >>  person's clothing, the role they are playing in the gaming
> >>  convention....?? I know there is an answer in the case of name
> >>  badges, but the point is that this answer depends on an external
> >>  convention known to the users, an implicit shared set of 
> assumptions,
> >>  a background. It's not inherent in the very idea of a 
> name badge. You
> >>  CAN interpret name badges differently, and I expect some 
> symposia do.
> >>  And as soon as you allow this kind of contextuality, you lose
> >>  uniqueness of denotation. But according to what you say 
> here, it is
> >>  *logically impossible* to mis-interpret a URI.
> >
> >(Sorry Graham for continuing to jump in here, I can't help myself ;-)
> >
> >I myself wouldn't argue that it is impossible for overloading to
> >occur, where different *users* presume a different denotation, and
> >eventually, we can hope to have sufficiently intelligent SW agents
> >to be able to identify and even cope with such overloading, but
> >from the viewpoint of a SW agent which is not sufficiently 
> intelligent,
> >it has at present no recourse to do anything but presume that every
> >time it sees the same URI that it denotes the same thing. If
> >overloading occurs, that may result in inferences that do 
> not correspond
> >to the real world. Cest la vie. The users then need to fix the
> >problem, which is the overloading of denotation.
> Actually I kind of agree, with caveats, at the present stage.


(it won't be the first time we've been in violent agreement ;-)

> >
> >Thus, from the SW agent's perspective, yes, it seems as if it is
> >logically impossible to mis-interpret a URI, since a URI would
> >be presumed to have a single, globally consistent denotation.
> >
> >Looking at the system from the outside, as a user, however, then
> >one can say that insofar as the specifications/presumptions of
> >the denotations are concerned, mis-interpretation by the users,
> >and subsequent mis-use by users, is certainly possible.
> >
> >One simply needs to keep clear whether it is the SW agents within
> >the scope of the SW machierny or the users outside the scope 
> of the SW
> >machinery which are doing the interpretation. SW agents cannot
> >mis-interpret, since they have no access to the 
> sub-primitive machinery
> >used to define URI to resource mappings. Users can however 
> mis-interpret and
> >subsquently introduce those mis-interpretations into the SW as
> >overloading of denotation, which results in ambiguity that can
> >cause unexpected inferences.
> >
> >>  Here's how this manifests itself in a web formalism, like RDF.
> >>  Suppose I make some RDF (or whatever) assertions about a 
> thing using
> >>  its URI. If there is a single referent, it must be the 
> same referent
> >>  *in all possible interpretations of my assertions*. So 
> what I write
> >>  just IS true or false of that thing, and a reasoning 
> engine ought to
> >>  be able to find out which *just by looking at the URI*.
> >
> >I don't see how it can find out anything by just looking at the URI
> >in isolation, since that URI is a primitive and it has no access
> >to the machinery by which the URI is mapped to the actual thing.
> Use any machinery you like. You still won't get to the actual thing 
> if it is something like a book, person, company or namespace.

I fully agree.

> >
> >>  But of
> >>  course it can't possibly find it out in that way, in 
> general, even if
> >>  we allow it to use Web machinery on the URI; and the 
> reason it can't
> >>  is because this assumption is completely false: what a URI denotes
> >>  *depends on the interpretation*, just like names and referring
> >>  expressions in all the other languages and notational schemes ever
> >>  invented.
> >>
> >>  This point is made even stronger if the things that one 
> GETs by using
> >>  a URI are considered to be representations of resources, 
> since then
> >  > the meaning of the representation depends on which semantic
> >>  conventions are applied to it.
> >
> >Why? If you have a URI, and that URI denotes a resource, but you have
> >no idea what that resource is, and you give that URI to an 
> HTTP server
> >as part of a GET request, there is no guaruntee whatsoever that what
> >you get back will help you in the least in figuring out what resource
> >is actually denoted by that URI.
> I didnt say it would. My point was that the semantics of the 
> embedding representation (on the page that is GETted) is what 
> determines the referent of this use of the URI, not some global URI 
> reference mapping.

I've never asserted that the URI to resource mapping is anything
that exists in any formal sense. It is something interned in the
minds of the human users, who use URIs to refer to things they
want to talk about. 

And yes, the way that different humans communicate what things
their URIs denote may take many and varied forms, most of which
will be non-formal, based on natural language and other forms
of expression.

And yes, humans interpreting a representation provided by an
HTTP server might be able to grok what resource is actually
denoted by that URI -- but only if the author of that representation
actually cares about communicating information about the actual

Most representations on the web do not contain information about
denotations, because actual denotations of URIs are not critical
to Web applications. All Web applications care about are getting
consistent *representations* -- and most folks don't even think
in terms of resources or representations but of documents, files,
data streams, etc. that are "accessible" via a given URI.

This whole issue of what a URI denotes and consistent denotation
and distinction of denotation between things and their
representations, etc. is all of minimal importance to the *Web*
but of critical importance to the SW. 

> >
> >Insofar as the Web is concerned, that doesn't matter. Folks just
> >want to get useful content and are not concerned with the level of
> >precision of denotation that the SW requires. It's not 
> surprising that Web
> >users frequently confuse the denotation of URIs between resources
> >and representations of those resources, or even other things
> >indirectly referred to or depicted in the representations.
> >
> >But just because (a) the Web does not need the level of precision
> >that the SW needs, and (b) just because human users of the Web
> >can deal with overloading of URI denotation, does not mean that
> >the SW should either not care about overloading of denotation or
> >be expected to be able to deal with it (at this stage).
> I largely agree about that.



Patrick Stickler, Nokia/Finland, (+358 40) 801 9690, patrick.stickler@nokia.com
Received on Tuesday, 29 April 2003 04:42:05 UTC

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