W3C home > Mailing lists > Public > www-tag@w3.org > July 2003

Re: resources and URIs

From: pat hayes <phayes@ihmc.us>
Date: Wed, 16 Jul 2003 18:38:22 -0500
Message-Id: <p06001226bb3b84fb7a17@[10.0.100.23]>
To: Tim Bray <tbray@textuality.com>
Cc: www-tag@w3.org

>pat hayes wrote:
>
>>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.
>
>The first half is correct; if it has a URI it's part of the Web, if you
>can get representations it's a more useful part.  What the resource *is*
>in this example is not specified.
>
>If it pleases you to assert that the URI identifies "Oaxaca's weather
>forecast according to the example.com online weather service" then this
>is a useful representation.

Its not a matter of what pleases me, but what your document is 
saying. Are you saying it is OK for me to consider the weather as an 
identified resource in the sense used in the TAG architecture 
document? I have no way of telling. On the face of it, however, I 
would find it very hard to reconcile that interpretation with the 
idea that a resource must be 'part' of the Web. I really cannot 
understand what it could possibly mean to say that weather was part 
of an information network.

>For the Web to behave well, we should be
>aware of what the owner of example.com asserts that this resource is,
>although this doesn't really constrain the way we interpret any
>representations we get back.
>
>However, it is also clear that neither the weather in Oaxala, nor 
>Oaxala itself, are entities of this kind:  weather and cities in 
>Mexico are not the kind of entities which can be thought of as 
>'objects on the networked information system'.
>
>On the contrary.  If the proprietors of example.com assert that
>http://weather.example.com/oaxaca is intended to represent the weather
>in Oaxaca, as an abstraction, for use in the development of a formal
>inferencing system about global weather, and its primary practical use
>is to retrieve slots from a KR repository, well, hey, that's OK too.

Sure, but that isn't what I said. I know that a URI can be used to 
denote weather, and that a URL can be used to retrieve a webpage 
which itself refers to weather: I use this latter fact all the time. 
Neither of these is the same as saying that the weather is an object 
on the networked information system.  That was my point: are you 
talking about being part of an information network or are you talking 
about being *referred to* by something on an information network? You 
seem to be confusing these ideas.

>In
>this case it would be surprising if the proprietor provided much in the
>way of representations, and if they did, they would likely be quite
>different from those provided if the operator asserted it was an online
>weather service.
>
>The Web Architecture doesn't provide a way to talk about what 
>resources are or what they mean.

Why does it even need to mention resources at all?

>  For the moment, we're stuck with human language for that.  The Web 
>provides a way to name them and a way to move representations 
>around.  That's all the Web of today needs.

I think that is all the Web will ever need, in fact. That is all the 
SW needs as well: it just uses different kinds of representation.

So resources are just sources of representations, then? Can I make 
that assumption? If so, the weather is clearly not a resource.

>The semantic web, one would hope, will provide a way to talk about 
>what resources mean and the relationships between them.  But it had 
>better be able to work usefully with the Web as it is.
>
>>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 as 
>>http://chandra.harvard.edu/photo/2003/ngc1068/index.html were to 
>>include RDF markup, one might expect to find things like this in it:
>
>This is all very well and good, but it is neither necessary nor 
>appropriate that we pretend that the current architecture of the web 
>comprises any notion of "about-ness".

Fine: but then please don't talk about semantics in the architectural 
description. Why do you even need to mention resources which the 
representations are 'of' ?? All the architecture needs to do is to 
ship representations around. Leave what they are 'of' to a semantic 
theory.

>It has URIs, representations, and protocols to move them around. 
>You may not like this, but you have to deal with it, because that's 
>just the way it is.

I like that perfectly. If the TAG architectural document only talked 
about URIs, representations, and protocols then I would be extremely 
happy.

>>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 as equivalentTo and FunctionalProperty to state -- 
>>or at least claim -- formally that two URIs identify the same 
>>resource. "
>>
>>is incorrect. These assertions claim that two URI references 
>>*denote* the same entity in all interpretations. That is not the 
>>same notion as 'identify'.
>
>I think we'd be happy to accept corrections if our statements about 
>what SW technologies do are incorrect.  To be useful, it will have 
>to be more elaborated than that above, because I suspect that most, 
>like me, will not have any idea what the differences are between 
>identification and denotation.

Fair enough. Unfortunately Im not entirely sure what 'identify' means 
either, only that it cannot be the same as 'denote'.

>>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
>
>So what?  It is a correct and important assertion (and one that 
>needs to be made) that you're not part of the Web until you have a 
>URI, and that being a part of the Web is a good thing.  If there is 
>a problem with taking it literally in the SW context, then don't 
>take it literally in the SW context.  This worries me, because the 
>statement is correct and useful.

There is no way to know if it is correct or not since it doesnt seem 
to mean anything that can be tested for truth.  Am I identified by a 
URI? How could anyone possibly tell? There are URIs for my webpage 
and for my email address and probably for many other things somehow 
related to me.  Am I part of the Web? If not, how can I get myself to 
be part of the Web? Once I am a part of the Web, will I feel 
different? Will it hurt?  Etc. I have absolutely no idea how to 
answer questions like this, even after reading every available 
document specifying the architecture of the Web.

>>, 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.
>
>I don't understand the above.  URIs in the Web architecture are what 
>they are, and what they are is effectively defined by a huge 
>universe of deployed technology; they are character strings that can 
>be used to look things up in databases and to retrieve 
>representations.

Fine.  Notice that you did not mention resources or use the word 
"identify" in that sentence.

>  This is the basis of the Web architecture.  If you want to use them 
>as a basis for building the SW, that's fine, but please don't try to 
>stop us from writing down an accurate description of reality as it 
>empirically is.
>
>>We need to get clear on this issue, or else we will continue to be 
>>mired in confusion.
>
>We are *entirely* clear what URIs are in the context of the 
>Web-that-is, and we have the software to prove it.

Software does not establish clarity or understanding.  But OK,  I am 
operating on the assumption that *you* have a clear idea what URIs 
and resources are; my point is only that it does not come across very 
clearly in the current document.

>  We are trying to write down that shared understanding.
>>
>>Let me suggest that it would be worth distinguishing between what a 
>>representation is *about*, and what resource *produced* it.
>
>This sounds like an excellent job for the Semantic Web.  The current 
>Web gets along fine without such a distinction.

I don't think it does, in fact. That is, I think that it is necessary 
to make this distinction even to make proper sense of examples like 
the weather in Oaxala. If this distinction were not made, websites 
like http://www.wunderground.com/US/FL/Pensacola.html would be 
incompatible with the current TAG account of Web architecture, or 
else weather would have to be part of the information network. If 
this is true, then please tell me what transfer protocol  and MIME 
type I should use for accessing the weather on the Web: not 
information *about* the weather - as you have made clear, anything to 
do with aboutness is not part of the architectural description -  but 
the weather *itself*.

(I suspect, if I may be ad hominem for a second, that your response 
can be understood as a rejection of any kind of semantic analysis, 
perhaps going along with the sense that semantics only becomes 
relevant for the SW, and is irrelevant to the current (non-semantic?) 
Web. If so, this is a serious mistake. Semantics is centrally 
relevant to the Web, because the entire point of the Web is to convey 
representations form place to place, and it is only semantics which 
makes a representation distinguishable from random noise. The very 
idea of a 'representation' includes semantics: a representation which 
doesn't mean anything isn't a representation.)

>>The document currently says that URIs are used to retrieve 
>>representations 'of' a resource.  It is easy to read this as saying 
>>that the representation is 'about' the resource: that it 'refers 
>>to' or 'describes' the resource; but this is evidently incompatible 
>>with the notion of a resource as something that must be 'part of' 
>>an informational network.
>
>You can read anything you want into it, but it doesn't actually say 
>anything more than what you describe in the first sentence.  Nor 
>should it.

Fine, but what IS that sentence saying? What do y'all mean by "a 
representation of" a resource? In both the informal English senses 
and any technical sense I know of, that has to mean that the 
representation *represents* the resource. Apparently you do not mean 
that. So what DO you mean?

Pat Hayes



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Received on Wednesday, 16 July 2003 19:38:27 UTC

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