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Re: resources and URIs

From: Dan Connolly <connolly@w3.org>
Date: 16 Jul 2003 17:51:37 -0500
To: pat hayes <phayes@ihmc.us>
Cc: www-tag@w3.org, Pat Hayes <phayes@ai.uwf.edu>
Message-Id: <1058395896.2422.438.camel@dirk.dm93.org>

On Tue, 2003-07-15 at 18:20, pat hayes wrote:
> Gentlemen, I would like to ask you to please clarify the meaning of
> the terms 'resource' and 'representation' in
> http://www.w3.org/TR/2003/WD-webarch-20030627/.

It seems to me that your request is pretty much a request
to resolve the httpRange-14 issue.

We would sure like to. We've been working on it, slowly,
since July 2002 (and who knows how long related
discussions that predate the TAG issue have been going on).
We tried ignoring it and hoping
it would go away for a while (Sep 2002 to Feb 2003)
but that didn't work, so we put it back on the table.
But we haven't made much progress on it since.

That said, a few comments from my perspective on the issue...

> Allow me to elaborate.  Your introductory example asserts the
> following:
> "Objects in the networked information system called resources are
> identified by Uniform Resource Identifiers ( URIs ). "
> and later the document says:
> "URIs identify resources. When a representation of one resource refers
> to another resource with a URI, a link is formed between the two
> resources. The networked information system is built of linked
> resources, and the large-scale effect is a shared information space.
> The value of the Web grows exponentially as a function of the number
> of linked resources (the "network effect").  "
> These, and other pieces of text concerning 'resources' published by
> other W3C authorities,  seem to clearly indicate that the word
> "resource" is intended to refer to the entities *in* the networked
> information system: they are the kind of thing we use words like
> 'website', 'client' and 'server' to describe; they are things with a
> computational state, things with which one can communicate, things
> which send and receive information which can be transmitted along
> optical fibers and twisted pairs, things than can be linked to one
> another.

By 'resource' I mean not only those, but also lots of other

> So far this is clear; and the account of 'representation' given in the
> document is also then reasonably clear:
> "Agents (such as servers, browsers and multimedia players) communicate
> resource state through a non-exclusive set of data formats, used
> separately or in combination (e.g., XHTML, CSS, PNG, XLink, RDF/XML,
> SVG, SMIL animation). In the travel scenario, Dan's user agent uses
> the URI to request a representation of the identified resource. In
> this scenario, the representation consists of XHTML with embedded
> weather maps in SVG. "
> On this picture, the information (which Dan, in your introductory
> example, reads on his screen, and which is in some sense all about the
> weather in Oaxaca) is a representation of the (current state of) some
> entity *in the WWW itself*: a resource in the global information
> network: the state of some computer system, or maybe some abstraction
> of a computer system. 
> However, it is also clear that neither the weather in Oaxala, nor
> Oaxala itself, are entities of this kind:

it is?

>   weather and cities in Mexico are not the kind of entities which can
> be thought of as 'objects on the networked information system'.

Yes, they are; I think of them that way.

>  Other examples abound,
> eghttp://chandra.harvard.edu/photo/2003/ngc1068/index.html  is in
> clearly about a galaxy containing a supermassive black hole, which is
> also not something one would expect to find as part of an networked
> information system, given the likely physical constraints on network
> architecture. 

I think that particular identifier refers to a document about
a galaxy, not the galaxy itself; if you want to refer to
the galaxy itself, you should use a URI with a # in it.
[folks with other opinions on httpRange-14 disagree,
I believe.]

> It seems that there is a systematic ambiguity between two senses of
> 'resource' (or maybe two senses of 'representation') here. In your
> first example, I doubt very much that Dan, when looking at his screen
> after telling his browser to retrieve
> http://weather.example.com/oaxaca, thinks of what he is reading as in
> any sense about the state of something on the WW information network.

If 'Dan' refers to me, I can say with authority that I do.

In particular, I think http://weather.example.com/oaxaca refers
to a document about the weather, not the weather itself.
But http://weather.example.com/oaxaca#weather might refer
to the weather itself.

>  Certainly if I were in his shoes, I would be reading it as being
> about Oaxala and weather: that is why he is reading it, presumably: to
> find out something about the weather in Oaxala.  So what this
> representation is *about* is not, apparently a resource:

In this particular case, the representation is a representation
of a document about the weather.

>  so it is not a representation of a resource, in the usual sense of
> 'representation' and what is apparently your sense of 'resource'.
> Similarly,http://chandra.harvard.edu/photo/2003/ngc1068/index.html
> sure reads to me like it is about NGC 1068. But this means that either
> it is a 'representation' which is not about what it is 'of', or else
> that NGC 1086 is an 'object in the networked information system';
> neither of which seem to me to be remotely plausible as factual claims
> using the ordinary senses of the words, and kind of brain-damaged as
> attempts at a formal definition of some kind of architectural/semantic
> theory.

No, it's pretty plain:
The URI http://chandra.harvard.edu/photo/2003/ngc1068/index.html
refers to (i.e. denotes in many/most interpretations)
a document about a galaxy.

> Now, this could be just a matter of philosophical opinion, were it not
> for the fact that semantic web languages like RDF and OWL have been
> given *formal* semantic theories which have direct architectural
> consequences for Web agents, and which depend crucially on notions
> like the term 'about' I have used rather loosely above.  RDF uses URI
> references as *names* to *refer* to entities. So if a web page such
> ashttp://chandra.harvard.edu/photo/2003/ngc1068/index.html were to
> include RDF markup, one might expect to find things like this in it:
> <rdf:Description
> rdf:about="http://chandra.harvard.edu/NGC/ngc1068"
> rdf:type="http://chandra.harvard.edu/AOtype/Activegalaxy7"
> </rdf:Description>

That would be... wierd. Try:

<http://chandra.harvard.edu/NGC/ngc1068> foaf:topic
  [ rdf:type [
    is foaf:topic of 
      <http://chandra.harvard.edu/AOtype/Activegalaxy7> ]].

using foaf:topic (cf http://xmlns.com/foaf/0.1/)
which is kinda like containsInformationAbout-Focally
in the cyc vocabulary. In full RDF/XML glory, that's...

<rdf:RDF xmlns:foaf="http://xmlns.com/foaf/0.1/"

    <rdf:Description rdf:about="http://chandra.harvard.edu/AOtype/Activegalaxy7">
        <foaf:topic rdf:nodeID="_g1"/>

    <rdf:Description rdf:nodeID="_g0">
        <rdf:type rdf:nodeID="_g1"/>

    <rdf:Description rdf:about="http://chandra.harvard.edu/NGC/ngc1068">
        <foaf:topic rdf:nodeID="_g0"/>

> where the URIs refer respectively to a galaxy and an RDFS class of
> galaxy types. This is completely incompatible with what your document
> says about resources and representations.

No, it's just a wierd way to model it... you're saying that
a galaxy is something you can get at with the HTTP
protocol. You don't have to do that.

>   Using the URI in this way does not create any kind of link between
> anything on this planet and NGC 1086 (which is, fortunately, about 50
> million light-years away).  But RDF/RDFS/DAML/OWL/OIL and all the
> other emerging Semantic Web formalisms *require* that URIs be used in
> this way,

nope. See counterexample above.

>  as *referring expressions*, not as informational links in a global
> architecture.
> The RDF/RDFS/OWL semantics assumes that URI references refer to
> "resources" , but it explicitly denies that this word "resource" is
> limited to the kinds of resource that you seem to be talking about. On
> the SW view, *anything* is a resource: galaxies, regions of France,
> kinds of wine, sodium atoms, classes, mathematical abstractions, even
> fictional entities: anything that can be referred to by a name

Yup. And I'm pretty sure nothing in the webarch doc says otherwise.

> . None of these can possibly be "objects in a networked information
> system".


>   So whatever you are talking about, and whatever they are talking
> about, y'all cannot possibly be using the words "resource" and
> "representation" in the same sense.
> As a result, several of the assertions you make in this document are
> not correct. For example
> 2.8.2
> "merging Semantic Web technologies, including "DAML+OIL" [ DAMLOIL ]
> and "Web Ontology Language (OWL)" [OWL10 ], define RDF properties such
> asequivalentToandFunctionalProperty to state -- or at least claim --
> formally that two URIs identify the same resource. "
> is incorrect.

I don't think so.

>  These assertions claim that two URI references *denote* the same
> entity in all interpretations.


>  That is not the same notion as 'identify'.

Well, it's pretty close... try taking 'identify' to mean
"denote in many/most useful interpretations".

The a claim that they denote the same entity in all
interpretations is also a claim that they denote
the same thing in many/most useful interpretations
(though it doesn't work the other way around).

> In fact, there is no such notion as 'identify' in RDF/RDFS/OWL
> semantics; and the first principle in section 2 ("All important
> resources SHOULD be identified by a URI ") is meaningless when taken
> literally in the context of semantic web languages, as URIs there
> typically cannot be said to identify anything: they act as names whose
> possible referents are constrained by the assertions made using them,
> but they are not 'linked' to anything, not 'bound' to anything, and
> are not obliged to 'identify' anything; and the universes of discourse
> may contain entities which cannot possibly be all identified or even
> referred to by URIs, since there are too many of them, or it is
> physically impossible to identify them with enough precision, or
> simply because it is impractical to do so.
> ------
> Sorry this comes across so negatively, but there seems to be a central
> misunderstanding right at the center of several architectural accounts
> of the Web, and I think it is important to get it sorted out.
[... I don't see any discussion of the text in the architecture
document below here...]

I agree that talking about formal systems and distributed
systems in the same document is tricky... my intuition
is that formally, the same URI can identify different
things in different interpretations. But there's
a sort of economic/architectural benefit to having
lots of interpretations agree. Or something... I'm
still trying to figure out how to say it.

Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Wednesday, 16 July 2003 18:51:41 UTC

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