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Re: statements about resources vs. representations

From: Pat Hayes <phayes@ihmc.us>
Date: Fri, 21 Nov 2008 14:36:38 -0600
Cc: Harry Halpin <hhalpin@ibiblio.org>, Alan Ruttenberg <alanruttenberg@gmail.com>, "public-awwsw@w3.org" <public-awwsw@w3.org>
Message-Id: <04358C7B-9428-44B1-9983-43EB8FA69DC3@ihmc.us>
To: Jonathan Rees <jar@creativecommons.org>

On Nov 21, 2008, at 2:17 PM, Jonathan Rees wrote:

> On Nov 21, 2008, at 1:39 PM, Harry Halpin wrote:
>> Alan Ruttenberg wrote:
>>> AFAIK there is no way to make a statement about a representation,  
>>> only
>>> about a resource. Therefore we can not evaluate the truth of  
>>> something
>>> like a statement involving containsWord solely by looking at
>>> representations.
>> Furthermore, I was under the impression that this inability to speak
>> about representations is a "feature", not a bug, since one assumes  
>> that
>> representations in of themselves are too ephemeral for someone to  
>> *want*
>> to make statements about them.
> But AWWW and RFC 2616 talk about representations, and we do too.
> We have to in order to talk about the semantics of HTTP.
> Just because some people don't want to talk about them doesn't mean
> such talk should be prohibited - that would be unscientific and  
> undemocratic.
> And of course they're exceedingly important. If all representations
> were to disappear from the earth, the Web would cease to exist. So
> it seems not only permissible, but important to talk about them.

Hold up. I think what Harry should have said is that they are too  
ephemeral for someone to want to give them an enduring name or  
identifier. But there are other ways to refer to things than baptizing  
them with a URI for all time. For example, one might refer to a  
representation by specifying the resource that emitted it (which has a  
URI, by assumption) and the exact time it was created (a literal), and  
the "name" of the resource would then be a good old RDF blank node. No  
URI involved, but you can still talk about it.


> The intermediate states in the AMD K5 are even more ephemeral than
> representations, but they are named and reasoned about using formal  
> language,
> and participate in formal proofs. I thought this was the kind of thing
> RDF and OWL were supposed to be for? Maybe they don't have as rich a  
> proof
> theory as ACL2, but the whole reason I've been attracted to them is  
> that
> they are *general purpose* formal languages. So nothing should be  
> off limits.
> On the other hand, I introduced the notion of a fixed resource
> defined by a representation to appease those who hate making
> a representation the subject of a triple. Since representations
> and fixed resources are "isoontic" (David MacAllester's term),
> and are even confused in Tim's memo, I don't see what the big
> deal is, but there's no need to aggravate anyone by using
> representations when fixed resources will do.
>> More below:
>>> -Alan
>>> On Fri, Nov 21, 2008 at 8:58 AM, Jonathan Rees <jar@creativecommons.org 
>>> > wrote:
>>>> (Using "representation" in the AWWW sense here.)
>>>> Suppose I have a resource R, and for some reason I believe that
>>>> R dc:creator author:Charles_Dickens.
>>>> Now suppose that I do a GET to obtain a representation, and let F  
>>>> be
>>>> the fixed resource (see [1]) whose representation is this  
>>>> representation.
>>>> (I'll need a term for the coercion of representation to fixed  
>>>> resource, so
>>>> I'll say "the FR of the representation.")
>>>> Assuming good faith and proper functioning on everyone's part,
>>>> can I conclude that F dc:creator author:Charles_Dickens . ?  I  
>>>> suspect
>>>> so, but is this idea codified anywhere? Wouldn't this be part of  
>>>> AWWSW?
>> You have no choice, as you can't talk about the representation.
> You seem quite sure of this. What is your evidence for this statement?
>>>> It seems to me that some properties will be shared between a  
>>>> resource
>>>> and its representations' FRs, while others aren't.
>> Ah, this is a problem, one I think the HTTP in RDF draft is working  
>> on.
> Hmm, I missed that, will look.
>>>> E.g. a property containsWord could easily be true of one  
>>>> representation
>>>> but not another (e.g. if the representations differ by language).  
>>>> Or,
>>>> more obviously,
>>>> one can meaningfully talk about the media type and content-length  
>>>> of a FR,
>>>> but not necessarily of its originating resource. Volatility is  
>>>> similar: the FR
>>>> is by definition not time-varying, but the resource may be.
>>>> I guess this is what Tim's "generic resources" memo [1] is saying.
>>>> Are there any properties of a resource that can be inferred
>>>> from its representations? That is, when I do a GET, do I
>>>> (or rather a stupid automated agent) learn anything
>>>> at all about what the resource is? I certainly don't learn anything
>>>> about, say, volatility, unless we're lucky enough to have
>>>> a credible assertion about it in the representation.
>>>> But I would guess that at least for things like authorship
>>>> (aspects of the content), if P and Q are disjoint classes,
>>>> and P applies to a resource's representation's FR, then you can  
>>>> conclude that
>>>> Q does not apply to the resource? That is, if you find that
>>>> any representation's FR's creator list consists of {George  
>>>> Eliot}, then
>>>> you know that the resource's creator list cannot be {Charles  
>>>> Dickens}.
>> It would seem like one has no choice but to infer the resource from  
>> the
>> representations!
> According to my reading of AWWW, it's up to a URI's designated  
> naming authority to determine what the URI denotes. (Well,  
> "denotation" isn't objective, so really what this is saying is that  
> the community is requested to respect, as best it can, what the  
> naming authority has to say on the question of what the URI names.)  
> So you should be able to find out what the URI names by establishing  
> communication with the naming authority.  We know that HTTP metadata  
> is authoritative [1], so HTTP might be one way to get information  
> from the naming authority, but there could be other ways, such as  
> calling them on the phone.
> If what you mean is that a[n information] resource is defined by its  
> representations - that you can't have two different resources that  
> have the same representations - that would be a very interesting and  
> powerful statement, similar to what David Booth has been advocating.  
> But how would one come to know what its representations are? Doing  
> GETs will only get you *some* of the representations, and some  
> aspects of the resource, such as its fixedness (if it's fixed),  
> depend on knowledge of *all* of its representations, many of which  
> may not exist until you and I are dust. In general, the naming  
> authority may have opinions about the meaning (referent) of the URI  
> that are completely consistent with all of the resource's observed  
> representations, but are not a consequence of them.
> Also, how would one account for POST? Resources X and Y could have  
> identical representations for all GET requests, but still differ in  
> that a POST to X would have a different effect than a POST to Y.  
> Certainly X and Y are not the same resource in this case, so GETs do  
> not the resource make. (You might have plausible deniability by  
> claiming that X and Y differ by some representation not obtainable  
> via GET, but I don't think you'd be taken seriously.)
> One could conclude that there is no knowing anything about any  
> information resource other than that some set of observed entities  
> are representations of it, but this seems awfully cynical. I prefer  
> the AWWW view, that the naming authority knows. This allows us to  
> ask, what would the NA say, and what would we rather it not say, if  
> it could speak RDF? And by extension, other speakers.
> There has to be some intersection between common sense - things like  
> Dublin Core in the wild, and Tim's assertion (if it is true) that  
> the resource named by http://www.w3.org/DesignIssues/Generic.html is  
> so well known that it needs no explanation - and HTTP semantics. If  
> we can't explain why, or when, it is legitimate to use http://purl.org/dc/terms/creator 
> , in a manner that's consistent with at least some of its uses in  
> the wild, then ... I will send unfriendly email.
> Best
> Jonathan
>>>> This doesn't hold for volatility: volatile and nonvolatile are  
>>>> disjoint.
> [1] http://www.w3.org/2001/tag/doc/mime-respect.html

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Received on Friday, 21 November 2008 20:37:57 UTC

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